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Understanding email deliverability

What is email deliverability, and what can affect it? Unpacking the complexities of what causes your emails to land in the inbox.



The world of email is growing at an alarming rate. Today, there are approximately 4.26 billion email users in the world, and that’s expected to grow to 4.73 billion users by 2026. With over half of the world’s population using email, mastering email is key to your business growth.

What is deliverability?

Deliverability is the rate in which your messages are being sent to the inbox of a given mailbox.

In order to have good email deliverability you need to have a great sender reputation, and that can only be done through a properly implemented email program.

Building an email program from the ground up is difficult even with an inhouse technical team dedicated to the effort. Many businesses opt for a little bit of help from email service providers to get the ball rolling with email experts driving the implementation of the program. Most of these programs only last a few months, and once everything is up and running your business is left to handle everything else from that point on. Easy, right?

Not exactly. While you might be sending fine out the gate, great deliverability isn’t a set-in stone metric. If your reputation starts to slip, it can land you on blocklists, in the spam filter, or worse – terminated.

So how do you stay on the right side of the deliverability equation? Here are common email deliverability practices that you must do that affect your reputation and sending.

Email authentication

Strong deliverability starts with authentication. Spammers often pretend they’re someone they’re not so, inbox providers take steps to ensure email senders are accurately representing themselves. Consider these authentications as your keys, you’ll be locked out of the inbox without them. These are the essential authentications:

  • Sender Policy Framework (SPF): SPF protects against spoofing by allowing domain owners to publish a list of IP addresses that can send email on their behalf.

  • DomainKeys Identified Mail (DKIM): DKIM is a cryptographic authentication method that proves an email originated at a specific domain and has not been changed during delivery.

  • Domain-based Message Authentication, Reporting and Conformance (DMARC): 

    DMARC is a specification combining both SPF and DKIM to determine the authenticity of a message.

Image shows how a DMARC policy works from an email with SPF/DKIM through the mailbox provider, to verifying DMARC implementation, passing DMARC authentication, and applying the DMARC policy to deliver to sender.

Authentications help defend your email program against fraud which improves your sender reputation, and by extension your deliverability. These protocols also allow receiving mailboxes to identify you as a trusted sender for smooth delivery.

Email lists

Email lists aren’t evergreen, in fact, most marketing databases degrade by about 22% each year. Degradation happens for a variety of reasons: employee turnover, lost jobs, full inboxes, death, and suspension of email services are just a few factors that play into your email list losing its quality. While it’s easy to run your list through a validation tool and remove full mailboxes and mis-entered addresses, you can’t account for people who aren’t interested in your emails anymore.

Those disengaged users bring your engagement metrics down, which hurts your reputation as a legitimate sender. If your reputation falls below a specific threshold, inbox service providers (ISPs) will move your messages directly to the spam folder or worse – block them entirely. If either happens, your recipients are less likely to read your messages and convert over to your website, decreasing the overall ROI of your email program.

For those uninterested users, oftentimes the best move forward is to sunset them from your list. The question is – where do you draw the line? There isn’t a one-size-fits-all solution to this problem, and it largely depends on how often you’re sending to that given list.

Determining the best route forward for sunsetting inactive or low engagement users is complicated, and several other core components should be visited before making any changes. Re-engagement campaigns and list segmentation strategies should be looked at before you remove any users, and these will largely depend on your business and sending needs. Regular list hygiene practices like these keep your engagement rates high and your email lists clean. Be cautious about implementing any of these practices, as it’s best to have an email expert who understands the intricacies of your email program aiding you in the execution of these clean up procedures.


DNSBL (DNS-Based Block List) and RBL (Real-time Block List) are lists of IP addresses that are suspected of sending spam and are used to prevent unwanted email messages from reaching unsuspecting recipients. It’s important to mention that the blocklists aren’t blocking your messages, but the inbox service providers themselves. These providers use this information from various blocklist services along with internal metrics to make decisions on whether or not to block a message.

Being listed on a blocklist isn’t necessarily going to cause deliverability issues, and while there are a lot of blocklists out there, not all carry the same weight to inbox service providers. Some providers are more important to Gmail, while others are favored more by Yahoo, it all depends on how inbox service providers view them.

That said, requesting de-listings is a delicate business. Senders might believe that the reason for the block is trivial or a mistake and request a delisting without changing their habits. Requesting a delisting in this way hurts your chances of future requests being accepted in the future, and you might receive a permanent block if not handled delicately. Email experts tend to have existing relationships with these blocklists and can help set you on the right path once they diagnose the issue.

Different inbox service providers, different tastes

Inbox service providers all have a different standard for legitimate email traffic, and they won’t go out of their way to tell you whether or not you have met said standard. Understanding specific needs for each inbox service provider takes a wealth of research and industry knowledge, and goes beyond your standard authentication protocols.

Inbox service providers check both your domain reputation and your IP reputation when your message reaches their servers, but each service provider weighs domains and IPs differently.

To add a layer of complexity, they won’t tell you which one they weigh more, so it’s difficult to know what to change if something goes wrong and all of your messages start bouncing or landing in the spam folder. If too many messages start landing in spam and hurting your reputation, entire subdomains and IPs can be tarnished for good, as building back that reputation can be a lasting effort on your part to fix with proper email best practice. It takes time to figure out what works and what doesn’t, and all of that can change on a dime if they decide to change their filters.

Feedback loops

As a side note, most major inbox service providers provide feedback loops through which they give you information about spam complaints. Signing up for these feedback loops is important to your overall deliverability as they track the negative feedback you’re receiving from your email recipients.

Signing up for these feedback loops can be time consuming, and it’s important to be aware if your email service provider signs you up for these automatically, or if you need to do it yourself. While it is negative, ignoring this feedback causes your reputation to plummet, and will cause your sending to be throttled or blocked completely. If you’re unsure about which feedback loops to sign up for, an email expert can help narrow down the list.

Scaling accordingly

When you first construct your email program, your infrastructure is set up for your current volume at that time. As your business grows, so does your overall sending volume, and your overall traffic might need some IP and/or domain segmentation restructuring.

In theory, if you’re sending over 100,000 messages a month you should be sending from a dedicated IP. Dedicated IPs are tied to one sender, rather than a shared IP that is linked to several senders at any given time. Nobody else is able to influence the reputation of a dedicated sending IP besides the sender using it, which in itself builds in a layer of reputation protection.

However, a fresh, dedicated IP has no reputation tied to it whatsoever, and this plays into both your benefit and detriment. The benefit being that your reputation is protected from bad actors. If you were to send all 100,000 email messages at once from that brand new IP, inbox service providers will flag your sending as suspicious. Slow and steady wins the race in this case, but ramping up your sending will take time and measured effort.

The manual warm-up process will look the same for IP and domain warm-ups in terms of the daily sending limits. Here’s a snippet of a warmup plan:

Daily sending limit numbers for warm-up process

Adapting to a changing landscape

Outside of maintenance and adjusting your sending for growth, remember: the email landscape can change in an instant. If GDPR isn’t evidence enough on that front, let’s revisit the Yahoo and AOL merger. Two providers suddenly had to function under the same system and rules after Oath acquired both brands. The merger led to a sizable group of senders being throttled by both providers, leading to hurt reputations all around.

In a similar vein, Gmail’s filters are some of the most sophisticated on the market, and they’re constantly changing their algorithms to combat spammers. While Google Postmaster Tools can inform a sender how their emails are performing, it’s not enough. It takes industry knowledge to truly set up your sending infrastructure for each and every inbox service provider.

Within our Deliverability Service, our customers see an average of 97.4% delivery rates compared to the industry average of 85% and non-deliverability service customer average of 94%.

How many of your emails are bouncing, and which IP and domain are sending those emails in the first place? You’ve landed on a blocklist, but how respected is that blocklist by inbox service providers? Do you submit a delisting right away, or do you wait? Day-to-day deliverability care is imperative for continued success, and keeping on top of it all is a job in itself, but imperative to your sending success.

Resources and final thoughts

Our team makes a point in understanding and learning about the changing landscape of email, and whenever changes occur, we’re sure to make the changes necessary to keep nothing standing between your messages and the inbox. Your email program doesn’t fit in a box, it fits in hundreds of thousands of mailboxes across the world, let’s work together to make sure you’re landing in the right ones.

Additional resources

Mailgun’s Deliverability Service is comprised of a team whose passion is understanding the intricacies of deliverability. Every day, our employees show what it means to be an enterprise email partner. We have the industry’s most established, experienced team, and we all share a goal: helping you succeed.

Learn about our Deliverability Services

Deliverability Services

Looking to send a high volume of emails? Our email experts can supercharge your email performance. See how we've helped companies like Lyft, Shopify, Github increase their email delivery rates to an average of 97%.

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