The Email Deliverability Guide

If you don’t follow our daily newsletter, you probably have little idea of what Stacked Marketer is and why we know anything about deliverability.

Well, Stacked Marketer is a daily newsletter for marketers covering breaking news, advice, insights, tips, tricks, case studies and reports for digital marketers. Whether Facebook, Google, Instagram, TikTok, Snap, e-commerce, affiliate marketing, email, CRO, SEO and more, we’re always filtering through the noise so you don’t have to.

Let’s give you a quick run down of why we know a thing or two about email deliverability.

  • We started this daily newsletter in April 2018 and since then we’ve sent well over 2M emails.
  • Our average open rate in the last 6 weeks before writing this (28th May 2020) has been 47.6% with an average of 14,000 subscribers.
  • We were completely new to email and made tons of mistakes along the way but we learned about things that make a difference that a big part of newsletters don’t use.

If you aren’t already subscribed, you can check out our newsletter here. It’s free, you sign up just with your email and if you don’t like it, you can unsubscribe. No hard feelings.

Table Of Content

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Who is this for and what you can expect to learn

Do you send emails somewhat regularly? Then this will be useful for you. If you never sent an email and don’t plan to, then we have to say this is obviously not so actionable for you but we don’t mind if you’re curious and want to read through.

We’ve tried to provide the action steps for important email practices such as:

  • Setting up Google Postmaster and explaining how to read the reports from it.
  • Using an email placement tool that tells you where your emails land (Inbox, Promotions or Spam) and analyzing the results to improve your open rates in the real world.
  • How to set up DKIM, SPF, DMARC and BIMI and why each of these matters.
  • How to set up a profile picture for your emails in Gmail, Yahoo! and other inboxes.
  • What is important when building HTML emails.
  • What are common phrases and content that can trigger spam filters.
  • Creating positive engagement signals that help inboxing.
  • How to switch email domains.
  • How to keep your list in top shape with good hygiene practices.

What we want to make clear from the very beginning is that these aren’t any secrets or tricks that magically get you to 50% open rate. Far from it… Please remember the following points:

  • All the advice we have here would fit better under a “best practices” banner than anything close to “secrets”. All the information is actually publicly available for free if you google each point. So the pieces of the puzzle are out there, we just wanted to bring you the finished puzzle.
  • We’ve only worked with email service providers (ESP), we never set up our own email server for this. If you use your own email server, what we’re discussing here is still very useful but you will have to handle more things yourself. In other words, where we say “the ESP takes care of it”, you will have to do that yourself.

We’ve also put together a small glossary for email specific terms you might now know yet, even though we tried to explain each within their own section. Let’s jump into the very important next chapter where we tell you what we think matters and what doesn’t.

Stacked Marketer was built to filter through the daily noise that exists in the marketing world. It’s a digital marketer’s 7-minute daily read, jam-packed with the latest news, trends, tech and actionable advice.

What matters and what doesn’t

If you only take away one thing from everything else in this guide, here’s what it should be: None of the practices in this guide will help if people actually don’t want to get your emails.

Everything that you see presented here helps improve your deliverability and open rates for emails people opted in for, emails that people want to get. It’s about avoiding any false positives for spam and promotions, it’s not about “tricking” the system.

If you are spamming, you will eventually get sent to spam, no way around it.

On that note here’s what doesn’t matter:

  • Single opt-in or double opt-in will not have a big effect on your deliverability if your content is what people want and you follow other practices.
  • Whatever email service provider (ESP) and tools you use… Almost all of them work fine, it’s not the ESP that makes or breaks your campaigns and their open rates.

Reaching the inbox is not clear cut. Gmail, the most used email service out there, is highly customized. You could go to one user’s inbox but spam for the other. We’ll explain how to encourage your email subscribers to provide the correct positive signals to improve your deliverability.

These will be useful for all inboxes but especially for Gmail, which is our main focus, seeing how ~65% of our readers use it.

Deliverability in general is not a clear-cut, black or white outcome. It’s a spectrum. The better you do it, the higher ratio of your emails get sent to the Primary inbox. This doesn’t mean you will never show up in Promotions or Spam. 

How Gmail deliverability works

One word – engagement. If we take things to the extreme, if the user is highly engaged with a sender (clicks, replies, opens, etc.), that sender will almost certainly be sent to Primary.

Depending on how “spammy” your content is, you might need a better engagement score. If you’re a known family member using text-only emails to send to one person, you are more likely to inbox than a non-opt-in bulk-sender with HTML-heavy emails. As we said, it’s not clear-cut.

We tried to make this guide as thorough as we can but you have to keep in mind a few things:

  • We don’t have insider info into how any algorithm is programmed so this is all purely based on our experience.
  • Some things can change, some things we didn’t run into because we’re not big enough. We don’t know everything but we shared as much as we know so far.
  • We are not tech people who know how to set up and maintain their own email server so all the advice comes from using an existing service. The advice is still good when you use your own server but we cannot help you set up Sendgrid or Mailgun or whatever else.
  • This is not about sequences and sales, this is about deliverability and open rates.

For every section we will make sure to mention why it matters and when we learned it’s useful. This way we build as much context as possible so you can judge for yourself if your situation is significantly different.

How to prepare

We’ll start with setting up the tools you need to track factors that impact deliverability. Sure, you will see your open rates changing if something is wrong but you won’t know exactly what’s causing it unless you have a look at all factors.

These are usually:

  • Your domain reputation
  • Your IP reputation
  • Your content, which is again split into:
    • Vocabulary and topics
    • HTML used to generate your email
    • Domains and links you use inside your emails
  • Authentication, aka proving that you are the real and rightful sender
  • Your existing list engagement

Some are very technical, some have more to do with general strategy for your email. Let’s first explain how to prepare yourself to monitor these factors, then we will go into detail for every single one.

Google Postmaster

As mentioned before, Gmail is by far the biggest so it’s key that you monitor your reputation with them in Google Postmaster.

In this tool you can see:

  • User-reported spam rate
  • IP reputation
  • Domain reputation
  • Feedback loop
  • Authentication
  • Encryption
  • Delivery errors

How to set up Google Postmaster

1. Authenticate your domain with DKIM or SPF. DKIM is the easier one and you can learn how to set it up in this guide too. Skip to the DKIM setup instructions by clicking here, then come back for the next steps. Don’t worry, we’ll explain DKIM in more detail too.

2. Go to

3. Click the “+” button on the bottom right.

4. Type in your domain and click Next.

5. Copy the TXT record Google gives you:

6. Go to your domain registrar and find the DNS settings dashboard.

7. Under your domain, add a TXT record with the Name/Key as the root domain (can be, @ or left empty, depending on your registrar) and the value or content should be the text you copied above.

8. Save the settings and return to Google Postmaster.

9. Click “Verify.”

It usually takes a few minutes but once your domain is verified, it will instantly appear in your dashboard.

How to read the stats

User-reported spam rate. Every time someone reports any of your emails as spam, it’s counted here (or should be, reports are never 100% accurate). One issue is that Google rounds up to a minimum of 0.1% if there’s any report. You want to keep this as low as possible. For what it’s worth, we get one blimp to 0.10% one day a month or so. Industry average is 0.02% according to Campaign Monitor. Keep it under 0.01% for a reasonable time frame like a month.

IP reputation. Unless you get your own email server or a dedicated IP, you send your emails from a shared IP, which the ESP takes care of. If you send enough emails at once, you might even see more than one IP in the report. What we found is that “Medium” or above are good enough for excellent deliverability. Gmail is smart enough to know ESPs must use shared IPs so, while IP is a factor, it’s not as important as the actual sender.

Domain reputation. This is simple, just keep it high. There’s no reason why your domain reputation, which is fully in your control should not be “high”. In some exceptional cases, the report on one day might fluctuate because of some reporting issues but that’s it.

Feedback loop. Only for large volume senders who implemented it. Usually, the ESPs take care of it so it’s not something we’ve dealt with and not something necessary if you use an ESP.

Authentication. Rather simple again, should show all authentications have 100% success rate. Some days might have small exceptions and reach 99% or so but that’s about it. It includes DKIM, SPF and DMARC. We will discuss implementing each of them in the more techie section of this guide that you can find here.

Encryption. While Gmail should like TLS, we haven’t seen any correlation between encryption and delivery. It’s more resource demanding and slower to use TLS so some ESPs might not force TLS.

Delivery errors. This gives you a report if your emails are rejected by Gmail. It was always 0% for us and should stay there. If you run into issues here, we recommend contacting your ESP support to see if it’s an issue on their end or with your recipients.

Inbox placement test: GlockApps

This is a paid tool. We’re not associated with them  We have an affiliate agreement with them where we get paid a commission if you sign up using our link. They are probably not the only ones that do this but they are the tool we’ve been using for over 2 years. It seems like the best value for the money and provides most comprehensive reports. If you want to use alternatives, go ahead. This is just a tool that served us well and we think it’s worth recommending.

What does GlockApps do?

  • Provides a placement breakdown (Inbox, Promotions/Others, Spam) for all popular email inboxes.
  • Spam scores for some of the most important spam filters.
  • IP reputation and sender score check.
  • Content analysis to identify possible spam phrases, broken HTML code, broken or blacklisted links.
  • Domain reputation analysis, identifying blacklisted or greylisted domains.
  • DMARC, SPF, BIMI checkers, bounce monitoring and a few other features that can be useful but that we don’t use given our situation.

We use GlockApps for every single newsletter we send out. Sometimes we need more than one test to tweak things but usually one is enough to just clean up the last few pieces of spam phrasing or replace any potentially black listed links.

This is how a test looks like.

How we analyse a GlockApps test

  • First, inbox placement. We check if there is anything off there, especially issues with Gmail placement being anything else but Primary.
  • We then check content analysis for blacklisted domains, broken HTML and links and spam phrases.
  • Last, we look at the spam filter scores and improve if the score is too high, depending on the flags we get. This is almost never an issue now because we got reasonably good at avoiding spam phrases and spam angles (Nigerian Prince, sex life, and so on).

How we try to fix issues that occur

  • We look at Gmail placement first. If it’s all or almost all Primary, there’s nothing else to do when it comes to this.

    • G Suite is for non-Gmail domains, for typical G Suite users.
    • Non-interactive is for emails that don’t interact with your tests (or emails) so it’s usually what a new recipient would see.
    • Interactive refers to emails that always open the tests and mostly have better placement results than non-interactive.
    • When all of Gmail shows a Promotions tab placement, we generally have a relative drop of 10% in open rate. For example, if our average is 45%, we usually have around 41% open rate when a newsletter is 100% in Promotions according to GlockApps.
  • We then look at the next biggest inboxes which are Apple, Outlook and Yahoo! 
    • We almost never had an issue with Apple and it was always fixed easily after a few days.
    • Outlook tests are somewhat less reliable. Certain topics and phrases trigger Outlook spam filters instantly. In practice, many of the emails are still delivered to the inbox, based on what we saw in terms of open rates when we had a Spam placement.
    • Yahoo! has always been one of the easiest and without issues.
  • We always try to fix the issues with placements, focusing on Gmail the most and we take these steps.
    • We implement all content suggestions from GlockApps: phrasing, links and any major angles they identify as an issue.
    • We test after we implement the suggestions.
    • If it did not fix it, we test with a generic subject line like the day’s date to see if the subject line causes the problems.
    • If it fixes it, we come up with new subject lines to test.
    • If it doesn’t we start testing each section of the newsletter, one by one.
    • Usually, one of the sections is what causes the issue and we heavily re-write it until it passes the test. In some cases, it’s the sponsored section which we cannot change so we sometimes send it out knowing it will land in Promotions for many. We do this because we know it still gets us over 40% open rate.
    • This won’t have a 100% success rate. Even with all the time in the world, we sometimes would not be able to identify and isolate the issue so there will be times when we just bang our heads against the wall. With practice, everyone gets better at it.
  • We highly recommend keeping notes and guidelines for phrases, topics and links to avoid in your emails.

Let’s look at an example where we did a fair bit of testing and in the end succeeded with improving. This was the newsletter of April 7th, 2020.

The result of our first test.

Clear issue: Promotions tab for Gmail

There were no obvious black listed domains we linked and not many spam words so we started testing bit by bit, starting with subject line. As explained above, the following test when there is such an issue is one with a neutral subject line.

OK, perfect, now we know it’s the subject line to blame. We had a 3-part subject line so we did a test with one bit of the 3-part subject line taken out. We couldn’t find the one to blame. Next we tested each part individually. We saw this one was with good inboxing.

Next we added another part to it to try to build our 3-part subject line again. This was the result. 

Promising! Then we added the 3rd part.

Damn! This created an issue. The upside is we know which part is to blame. Several tests later, we arrived at this version.

Not perfect but it’s much better than the first test. GlockApps is not perfect so when it’s in the vast majority showing Primary for Gmail, it’s good to go in our view.

If you do this with each of your campaigns, you can significantly increase your average open rate.

We got better so we rarely have spam phrases issues. Usually, we have 1-2 at most and those we can either fix easily or ignore because deliverability is still good enough.

Things we ignore:

  • Outlook in Spam (which is the red Spam part you see in the test above). It happens sometimes but we know most people still get it in their usual inbox if they use Outlook. Given the low amount of people who use this and the low impact, we’ve never bothered to figure out what causes issues with Outlook.
  • We also ignore if our ESP’s domain is greylisted or blacklisted in some tests. When we first saw this happen, we contacted support for Campaign Monitor, the tool we use. We saw that our emails were not suffering when it comes to inboxing and support explained that some databases are not reliable. They closely monitor and clean for all relevant blacklists.

IP blacklisting in GlockApps

As with domains, not all lists are as reliable as the other so what we usually do is ignore blacklisted IPs on GlockApps when they don’t affect our placement. We would recommend contacting your ESP about it too when you see a blacklisted IP and show them the database that is to blame.

If it’s important and it affects deliverability, they will handle it.


One big reason deliverability can suffer even when using good language and vocabulary is when you are linking to a blacklisted domain. There are two kinds:

  • Blacklisted website because they’ve been abused by some senders. This can happen to just about any website. We’ve gotten warnings in GlockApps tests from known domains like Social Media Today and Search Engine Journal. They are cleared quickly, usually in less than 24 hours.
  • Link shorteners. They are abused so you should avoid using any link shortener in your email. Public and free to use ones are almost certain to send you anywhere but the Primary inbox. If really needed, use your own domain as a link shortener and make sure it stays clean.

In either case, you should not send an email with a blacklisted domain. It will affect your own domain’s reputation.

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Setting up the techie stuff

DKIM, DMARC, SPF, BIMI… These acronyms might look scary but they are generally pretty easy to set up. It’s some copy-pasting. You literally just need to know what to copy and where to paste it. For some information, you will have to contact your ESP, whether that’s Campaign Monitor, MailChimp, ActiveCampaign, Klaviyo, etc. 

We’ll explain what each stands for, why it’s useful and how to set it up, including screenshots where possible.


DKIM stands for DomainKeys Identified Email. Once we break out the acronym, it gets much easier, right? It’s an email authentication method that verifies the messages were sent from a legitimate user of the email address. Only the person who has access to DNS settings can set it up, therefore only the rightful owner of the domain.

How do you know if DKIM is set up? You can see in the received emails in Gmail by checking the information here:

  • With DKIM:

  • Without DKIM: 

You can also try a lookup on MX Toolbox.

How to set up DKIM

This is something you will have to look up in documentation of your ESP or ask their support because you need the key value from them. In general, the steps are:

  1. Get the Key Value pair from your ESP.
  2. Add this as a TXT entry in your DNS settings.
  3. Verify by sending a test email or a lookup. It’s that simple.

The important bit is copying the right information from your ESP. We’ve collected a few from common tools here:

The best way to find it is by googling “How to set up DKIM in (your ESP)”. If this doesn’t show any results, ask support.


SPF stands for Sender Policy Framework and it’s another authentication method. It’s used to prevent spammers from sending messages from your domain. 

Similar to DKIM, SPF also consists of DNS entries. In this case, while we can explain partially how to do it, they are even more based on each ESP so know that you have to do similar copy-pasting as in DKIM but with very particular data that only your ESP can provide.

If you use G Suite for your business, you can have a look at how to set it up for Gmail (which we recommend doing anyway).

You can set up more than one SPF so you can allow more than one ESP or email server to send emails on your before. For example, we have both Gmail and Campaign Monitor.

You can do SPF lookups here.

Contact your ESP to help you set it up.


DMARC stands for Domain-based Message Authentication, Reporting and Conformance so it’s not just an authentication method but also policy and reporting protocol.

With the policy part, you can tell what the receiving email server should do when it receives an email from a non-authenticated email. The options are “none”, “quarantine” and “reject”. When policy is set as “none”, the message is delivered as usual. “Quarantine” tells the email server to accept the message but send it to the spam folder. The “reject” policy tells the recipient email server to outright reject the message.

DMARC is also a reporting protocol so once you have it setup, tools like GlockApps can help you track who and when sends email using your domain.

Important tip (because we made this mistake too): After you set up a DMARC policy of “quarantine” or “reject” you absolutely must set up SPF for all tools that you use to send email. So that means G Suite must also be set up, otherwise, if SPF is not set up properly, the DMARC policy will be followed and your emails will be sent to spam or rejected.

You should set this up alongside SPF with the help of your ESP.


A newer entry on this authentication list, BIMI stands for “Brand Indicator for Message Identification” and it does two things.

  • It authenticates the email because a BIMI entry is also only the owner of the domain can set up in DNS settings.
  • It sets up a custom branded avatar/profile picture for your sender. This is not currently supported by all inboxes but it’s likely just a matter of time until it is adopted more widely. At the moment, BIMI gives you a branded avatar in several Oath brands like Yahoo! and AOL, plus Gmail is testing it as well starting this year.

Before you can set up your BIMI entry in your DNS settings, you must have DMARC set up with a “reject” or “quarantine” policy (remember the important tip above?) and you must prepare your avatar. You need a copy of the desired image in an SVG format and you must host it under a publicly accessible URL.

These steps can help you convert your existing logo to SVG.

Once you have your SVG url, you will have to set up a TXT entry with the name “default._bimi” and the value “v=BIMI1 l=your URL for the logo”

Once you have it set, you can look it up here. 

This is how it looks for us. 

HTML heavy emails and Gmail clipping

Long story, short: the more HTML you use in your emails, the more likely they are to be considered Promotions or Spam.

Think of it this way: The more your email looks like a message from a friend, the more likely it is to be sent to the Primary tab, at least at the beginning. You can build up a reputation with HTML emails too but it’s always a scale that you have to balance.

So, if you are starting off on a new domain and a new list, if you can use a text-only email, that might be the way to go. For most people though, this is not possible because text-only emails have no branding.

You will have to use HTML emails so you have to make every character count. We’re not kidding. Here’s why. Gmail has a limit of 102KB file size per email. Any email that goes over it will get clipped and you end up with something like this:

This is bad in several ways:

  • Your deliverability can suffer, especially if your early emails are like this. Emails over 102KB are never from friends and family, they are always from brands and likely promotional.
  • Your open rate reporting will suffer. Most ESP position their pixel at the very end of the email. It’s a 1×1 pixel GIF that, when loaded, reports an open. Since this is set as the last item in an email, whenever it gets clipped, the open is not reported, unless someone clicks the “View entire message” link or clicks another link in the email. This means your open rate data will be highly inaccurate. Not all ESPs have this issue but most will.
  • Last but not least, it’s just an ugly and incomplete email! Readers won’t see your whole message unless they click one particular link. If you are sending a sales email, just think how much of your message will not be viewed and how much worse your conversion rate will be.

How to avoid Gmail clipping

Don’t use drag-and-drop editors if you send relatively long emails that are more than two full screen scrolls. We’re being very conservative here. The problem is visual editors add significant HTML and CSS to your email.

Before you schedule your email campaign, you should certainly do a GlockApps test and also send yourself a test email. Make sure one of those test emails is heading to a Gmail account so you can check for any clipping issues.

Important: The 102KB limit is for the actual file size of the original HTML email. It has nothing to do with image size. So, you can have an image that is 105KB embedded in the email, it’s not a problem. 

What is good to keep in mind is that every single space in your HTML file will take up space so what you want to do in an ideal scenario is to minify your HTML template before uploading it to your ESP and using it.

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Non-techie stuff

Authenticating your domain and making sure your technical setup is in top shape is certainly extremely good but that won’t do anything if your subscribers don’t actually show positive signs when they receive your emails.

In this section, we will break down what are the positive signals readers can give your emails that significantly help deliverability and hitting that elusive Primary tab.

Aside from those positive signals, we’ll also go through some other tips, including list hygiene guidelines. None of them require any coding or technical knowledge, that’s why we added them in this non-techie category.

Whitelisting and dragging to Primary

A common one and something you should certainly request in your first email you send out, whether that’s a confirmation email or a welcome email after someone subscribes.

Whitelisting can mean different things but generally, adding you as a contact, as a VIP or literally as a whitelisted sender is what we mean. In any case, this is a good positive signal that the recipient wants to receive your emails in their Primary inbox.

For Gmail specifically, asking people to drag your email to the Primary inbox will help significantly too. It won’t be something that once done works 100% of the time based on our experience, so you should certainly remind people once every few weeks, depending on your sending frequency. We do that about once a month.


Yep, that CTR is not just to get people to your landing page but it’s an important positive engagement sign .

It’s also important to get this early on because your initial email dictates much of how your future emails perform.


The hardest to get but might move the needle the most. While you can have good deliverability without replies, we found they greatly help.

It’s up to you to get creative with replies. We also recommend that you ask for them regularly, similar to whitelisting and dragging to Primary.

Putting this together for a “Welcome Email”

You need to have this done from the early emails you send. Early engagement dictates future engagement so a good initial email has to be designed around this.

When starting out, we checked how other big newsletters like Morning Brew, The Hustle, and The Skimm did it. 

Later on, we found the balance that worked for us. We’ll show you two different versions of our welcome email and we’ll explain why we made the changes and how that affected our stats.

Old Welcome Email.

Current Welcome Email.

Let’s break down what we wanted to do with the old email:

  • We wanted to explain clearly when people would get our daily newsletter.
  • We wanted to ask people to move us to the Primary tab and whitelist us.
  • We wanted to gather information about readers in a survey.
  • We wanted to have people opt into our referral system to get incentivized to share the newsletter with more industry folks.

When we started out under the domain and WHAT THE AFF brand, this worked pretty well. We were growing slower and the early readers were very dedicated to regularly opening, reading, engaging, etc.

If you want to know why we changed the name and domain, you can read all about it in our 2019 Annual Report here.

So we managed to get away with this messy welcome email. Why do we call it messy? Because it tries to achieve too many things at once and is overloaded with links and images. As we said before, the welcome email should build the initial engagement.

We didn’t see the issues because our old domain,, had high engagement from the early days but we noticed major issues once we transitioned to our current one,

Here were the stats when we used this same email under a new domain.

Those are the stats for about one month after switching the domain. For most people, this wouldn’t be too bad of a stat but we know it can and should be better. A GlockApps test revealed that our welcome email was getting sent into the Gmail Promotions tab.

So we started testing different variations. New template, different text, different links, etc.

Here’s what we found:

  • Less is more, so focusing on the engagement is best.
  • Images hurt a lot. We ditched the GIF showing how to drag the email to Primary and left the logo as the only image.
  • We prioritized the click more. It’s an easy step with a high value.
  • We still left the option to give us feedback in the survey but in a much less important position.
  • Last but not least, after a few people mentioned it, we also did the obvious and instead of sending people to a funny picture on the click, we sent them to a page on our website that explains what they can expect from us again and makes them more familiar with our usual tone.

Welcome Email stats since then.

  • Almost 25% improvement in relative open rate for the email.
  • More than double the CTR.
  • And to make it clear this has a significant effect, here are the average stats for us before.
    • Before:

    • After:

You might be asking yourself “Why is there such a big difference in emails sent in this period?” 

After the rebranding and the transition to the new domain was complete, we wanted to do many more promos. We didn’t do any promotions before that specifically because we wanted to make sure our onboarding is as good as it can be.

Compressing images

There’s no way around it, images are way more common in promotional emails so using them is tricky. We minimized the images used in our Welcome Email and aside from that, we always try to find a balance between image quality and image file size.

The earlier you are in your reputation, the less you should use images and when you do, the more compressed they should be. It also helps to make sure they load fast enough when you compress them, since some mobile connections can be less stable.

GlockApps gives reasonably good feedback when it comes to images. For compressing them, we use Compress Or Die. This seems to offer the most options to balance quality vs size.

We do want to reiterate a big point here. If people absolutely love your emails and look for them in all corners of their inbox, you can get away with much more. If people open your emails for the images, go crazy. These are just guidelines for the average sender.

List hygiene

Your average results on your current list will dictate a big part of your reputation with new subscribers. So if you aren’t careful with your list hygiene to maintain high engagement, it won’t work long term.

No matter how good your emails are, some people will simply drop off. In our case, 33% of the unsubs are because people just aren’t in the industry anymore.

That’s why we heavily recommend to not get stuck into thinking about list size. Instead, focus on your average unique opens per email and make sure you grow that. 

We have two list hygiene procedures. One of them is targeted at new subscribers that are just inactive and the other one is targeted at people who drop off over time.

For new inactives

If someone doesn’t open any of the 6-7 daily newsletters they get sent a re-engagement email that asks for a click to confirm they want to continue getting our newsletter. Otherwise, they get purged from the list.

Here is what our email looks like. 

We’ve also done some research into how others do it.

An interesting fact is how both keep sending even though we did not click to confirm in any of them.

For old inactives

This is somewhat simpler. If someone doesn’t open any of their daily newsletters for ~2 months, we send a re-engagement email asking for a click to confirm again or the inactive ones get purged.

Here’s what our email looks for this purpose.

Procedure to switch to a new domain

Some of you might know that earlier this year (late February 2020) we renamed to Stacked Marketer and also started using a new domain, which was fresh and had no previous reputation.

The sad reality is that you cannot instantly start sending from a new domain and expect a similar result. The new domain is fresh and old subscribers never interacted with it so you have to take it step by step and “warm up” the new domain.

Here’s what we did:

  • Ask existing subscribers to click on a link that opts them into the new domain. Only the most active and avid readers would do this so the initial few hundred subscribers on the new domain will be our biggest readers that create great engagement.
  • For the whole week, we kept asking people who didn’t opt in yet to click if they wanted to transition to the new domain. This kept more and more active people switching to the new domain.
  • After 1-2 weeks, we started manually adding the most active openers to the new domain as well even when they didn’t manually opt in.
  • After about 3 weeks, most of our list was transitioned to the new domain and at this point the old domain started to hurt in open rates.
  • When the old domain started to drop to about a 15% open rate, we simply transitioned everyone to the new domain.

We also had everyone new directly on the new domain. This might not have been ideal. If you read our section about the Welcome Email, you know that our initial results were under par. In the end, we managed to solve it and with the new domain, our results improved across the board.

In short, take it nice and steady if you switch your domains. It’s easy to mess it up but it can also be an opportunity to start fresh and do things better. If your domain seems to be struggling, switching to a new one might be helpful.

If your domain is good but you want to change your name like we did, it can still be a chance to improve.

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Specifics for each inbox that we’ve discovered

Not all inboxes are the same, not when it comes to inboxing and not when it comes to styling and design either. Outlook and Gmail are two common inboxes that have some rather unique behaviours which we want to explain, as much as we know about them.


  • Most common for us, about 65% of our readers use it.
  • The one where inboxing is personalized the most probably. If you create a perfect connection with the reader, you almost always go in their desired folder.
  • One of the least obvious when it comes to spam filters. There’s no clear spam phrases that cause issues. It can either be a part of the email or all the email. Can only decide when testing bit by bit with GlockApps.
  • Pretty good with CSS when it comes to design, can use many of the same properties and styling options as a normal website. Warning on this though: If you use CSS that Gmail does not support in an HTML style tag, Gmail strips the whole tag from your document and you lose 100% of your styling. That’s why when building a template for Gmail, you should split up your CSS into non-Gmail specific and general (that is supported by Gmail as well).


  • Complete opposite when it comes to CSS compared to Gmail. Doesn’t even support rounded borders. Make sure you test carefully if your template looks the same or at least good enough on Outlook. In our case, the main difference is that our normal template has rounded corners in most inboxes, but square ones in most Outlook clients.
  • You need to have a styling reset code initially in your template if you wish to have proper padding and spacing.
  • When it comes to inboxing, it’s probably the most difficult and with the highest variation. We’ve had people explicitly whitelisting us and clicking the “never send to spam” option but still received future emails in spam. 
  • There are some geos where Outlook/Hotmail is king so you have to take extra care of it if your subscribers are there.

Wrapping up

If you went through all this, top to bottom, we have to say ‘Congratulations!’. You probably know more about email deliverability than many “email marketing experts” out there. Look, we’ve said it from the beginning and we’ll say it again: If nobody wants to read your emails, all these tips here won’t do much for you.

However, if someone actually wants to read your stuff, implementing all the advice we’ve put together here will definitely help you skip to the front of their inbox.

So, first focus on writing emails your target audience loves to read. Then do the more geeky stuff we talked about here.

For our part, whenever we find something new that can be helpful, we’ll definitely be updating this instantly. We do this because this exact guide is part of our internal documentation for anyone who joins The Crew, so it’s exactly what we do – nothing is hidden.

If you want to be among the first to know when we find new tips for email and stay up to date with all marketing breaking news, case studies, reports & more, check out Stacked Marketer.

We send an email every weekday that can put you up to date with the most important and relevant developments for marketing in just a few minutes a day, whether that’s Facebook Ads, Google Ads, SEO, affiliate marketing, e-commerce and much more.

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BIMI: Brand Indicator Message Identification, it’s a DNS record that is used to display a company logo inside an email inbox if the email is legitimate.

CSS: Cascade Style Sheet, a style sheet language used for describing the presentation of a document written in a markup language like HTML. In other words, it’s used to make a website (or email) look nicer.

DKIM: Domain Keys Identified Mail, an email authentication technique that allows the receiver to check that an email was indeed sent and authorized by the owner of that domain.

DMARC: Domain-based Message Authentication, Reporting & Conformance, a protocol that uses SPF (Sender Policy Framework) and DKIM (Domain Keys Identified Mail) to determine the authenticity of an email message. It allows senders to specify how unauthenticated emails should be handled.

DNS: Domain Name System, a hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities.

ESP: Email Service Provider, a company that offers email marketing tools and software.

SPF: Sender Policy Framework, an email authentication method designed to detect forging sender addresses during the delivery of the email. Best used together with DMARC, otherwise it has limited use.

SVG: Scalable Vector Graphics is an Extensible Markup Language-based vector image format for two-dimensional graphics with support for interactivity and animation.

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