Email Deliverability, Explained

Why some marketing emails make it to inboxes and others don’t.

Written by Hal Koss
Published on Dec. 01, 2021
Email Deliverability, Explained

Brett Hellman just wanted to send some emails. The startup he founded, a product for coworkers to exchange feedback over email, relied on it. So when he discovered that his users’ messages were suddenly landing in each other’s spam boxes, he feared his company was dead in the water. And he had no idea why it was happening.

“I was losing sleep,” Hellman, who had left a stable job at Atlassian to launch Matter, told me. “[It was] pure panic.”

Email deliverability — the process that ensures emails get to where they’re intended to go — is important for every business. And while Hellman eventually fixed his startup’s email problem, his experience navigating email deliverability (which he has blogged about in detail) serves as an example of how confusing and chaotic it can feel.

Whether it’s a tiny startup sending templated outreaches or a global corporation blasting product promotions, the success of many businesses hinges on adhering to email deliverability best practices.

What Is Email Deliverability and Why Does It Matter?

Email deliverability describes the ability to successfully deliver emails into recipients’ inboxes, typically by sending emails they want, expect and engage with, while avoiding block lists and spam filters. So much business is conducted through email that it’s important for everyone to apply deliverability best practices, like getting domain authentication, responsibly collecting email addresses, fine-tuning email content and regularly cleaning email lists.


What Affects Email Deliverability?

After you write an email and hit send, lots of factors go into determining if it ends up in someone’s inbox or not. The recipient’s mailbox provider is going to look at your track record before they let you in.

“The trickier part is that each mailbox provider … uses a slightly different algorithm to make their filtering decisions,” Lauren Meyer, chief marketing officer at SocketLabs, told me. “At a high level, they’re working to ensure each message they deliver to the recipients is, first of all, safe for [users] to see and view — so it’s not malware, phishing or anything fraudulent — and is wanted and perceived as valued by that recipient.”

Sending an email is not like making a telephone call over a landline. For starters, telephone landlines are common carriers — meaning, if someone places a phone call on one (assuming they’ve paid their phone bills), it’s guaranteed by law to travel through the telephone network without blockage.

The internet generally — and email specifically — doesn’t work the same way, though. Each network that participates in the transit of email gets to decide whether they’re going to deliver a user’s email successfully to the intended recipient. That’s because mailbox service providers don’t owe users anything.

“You really need to consider what the mailbox providers care about, and then reverse engineer that.”

A huge, publicly traded company like Google is willing to provide billions of people with free email because users are not the customers — advertisers are. Advertisers want to run paid ads inside Gmail users’ mailboxes because there is a large, engaged user base to which they can show ads. And the only way Google can amass that user base is to provide users with a good email experience.

A good email experience is one that doesn’t let unwanted emails through. That’s why so much junk gets rerouted into spam boxes or is outright blocked — because if that sort of correspondence regularly made it to people’s primary inboxes, they’d probably sign up for a different mailbox service provider.

Since mailbox providers want users to be happy enough to stick around and see ads, the trick for businesses to get their emails successfully placed into their recipients’ mailboxes is to align their deliverability practices with the mailbox service provider’s interests. That means only sending emails people want and expect to see — not emails people find annoying, irrelevant, unsavory or unsolicited.

Mailbox providers determine if a sender’s reputation is inbox-worthy by interpreting a number of signals. Good signals include a track record of sending emails that generate opens and clicks, get marked as important or forwarded to friends. It’s a bad signal if the sender has a history of sending emails that don’t get clicked on or get deleted without being opened.

“You really need to consider what the mailbox providers care about, and then reverse engineer that,” Meyer said. “It’s less about the automation and the tactics that are super technical, [and more about] how do you deliver mail that is engaging?”

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How Do You Ensure Email Deliverability?

Email deliverability problems can usually be detected by keeping close tabs on a range of metrics, including deliverability rate, opens, clicks, spam complaints and unsubscribes. If these numbers swing wildly overnight or even get significantly worse over the course of a few months, it’s likely a signal from the mailbox provider that the sender isn’t adhering to best practices.

“The vast majority of issues around deliverability are caused by the senders themselves.”

“The vast majority of issues around deliverability are caused by the senders themselves,” Andrew Barrett, senior director, global email deliverability and ISP relations at Braze, told me.

And while many of these senders aren’t bad actors, the fact that their emails aren’t noticeably liked by their recipients is what leads them into deliverability trouble.

Email Deliverability Best Practices

  1. Authenticate your domain.
  2. Choose the right IP and warm it up.
  3. Collect email addresses responsibly.
  4. Send emails people want, at the frequency they want them.


Domain Authentication

Basic email hygiene starts with clearing authentication protocols. Doing so helps establish that the sender is a legitimate, trustworthy person or business whose emails belong in inboxes.

There are three main email authentication protocols to know. SPF, or Sender Policy Framework, allows senders to alert mailbox providers as to what users are authorized to send emails from that IP address or email server. DKIM, or DomainKeys Identified Mail, provides an encryption key to ensure that an email wasn’t altered during its transmission. And DMARC (which stands for Domain-based Message Authentication, Reporting and Conformance) protects a sender’s email from being spoofed and lets them say how they would like unauthorized emails to be handled.

Getting an email authenticated in these ways is the basic first step to ensuring deliverability.

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IP Warming

If a business is launching an email program from scratch, it will need to warm up its reputation by gradually sending data signals to mailbox service providers indicating that it is trustworthy.

Even if a business has been around for a while and has a good reputation, it’ll need to repeat this process again if it ever switches email service providers (mailbox providers don’t really understand the concept of brands, Barrett said, so they only see “good sender” and “bad sender”).

Mailbox providers “can never completely discount the possibility that a bad actor who’s doing dumb or evil things with email suddenly appears fully formed out of the grass on this brand new IP that’s been quiet for the last three years,” Barrett said. “If you go from zero to 100 miles an hour in three seconds — that’s what spammers do — [mailbox providers] are not going to take the mail.”

“If you go from zero to 100 miles an hour in three seconds — that’s what spammers do — [mailbox providers] are not going to take the mail.”

The warming process for senders is to slowly trickle data that mailbox providers can measure, so they can get an idea of what you’re about. Essentially, senders are training the mailbox providers’ expectations of them. For this reason, it’s typically best for senders to make sure the emails they send during the warmup process are going to be opened and engaged with by the recipients.

To make matters more complicated, businesses use one of two kinds of IP addresses — either shared IPs or dedicated IPs. Shared IPs are when many different senders are pooled together into one group. The upside for senders with low email volume is that they can benefit from the sender reputation of the whole group. The downside is that their deliverability can be negatively affected by the bad actions of another sender in the group. Dedicated IPs, on the other hand, are typically used by businesses that send hundreds of thousands or more emails each week. It’s helpful for businesses that want more control over their sender reputation, but the warmup process may take longer.

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Permission and Preferences

Deliverability problems can often be avoided if senders resist the temptation to scrape the internet for email addresses or purchase them from third parties. Not only will these lists contain spam traps — deliberately incorrect email addresses seeded into the web by internet service providers and other organizations, meant to expose this sort of list-collecting practice — but they’ll be full of people who didn’t ask to receive emails from the sender in the first place. That means they’re likely to report spam or unsubscribe, hurting the sender’s deliverability.

What businesses should do instead is ask permission. Let users opt-in on their own accord and indicate if they want emails from you at all. If they do, let them choose what type of emails they want to receive (it’s unlikely they want every kind of email a sender has to offer) and how often they want to receive them. And do this all upfront.

“Far too many senders make the mistake of presenting recipients with other options only after they’re headed for the door,” Barrett said. “And by then it is far, far too late to salvage what had been a valuable, permission-based relationship. Instead, I think more senders ought to be servicing a clear and well presented preference center at the time of the subscription, and not when they’re raring to leave.”

Too many businesses focus on building the biggest email lists possible when they should concentrate instead on building email lists of people who welcome their emails and want to engage with them. A large list doesn’t mean anything if the emails aren’t getting seen.

For that reason, it’s common for savvy senders to regularly prune their email lists, removing recipients who are not engaging with their emails.

More on Email EngagementHow to Use Email Courses to Ramp Up Engagement


Engaging Content

It’s difficult for senders to get out of the spam box once they’re already there. So it’s important they frequently monitor their email metrics to staunch problems before they get too big.

Meyer suggests that businesses not only pay attention to opens, clicks, unsubscribes and complaints, but also adjust their email content and frequency if the data shows that users aren’t happy with what they’re receiving.

For some businesses, that might mean sending fewer emails. That requires the restraint to only hit send on messages that are genuinely valuable or interesting to the recipients, not just ones that are created simply to remind customers they exist.

For others, that might mean skipping out on the manipulative engagement tactics, such as writing misleading subject lines to juice open rates or making the unsubscribe button hard to find (such hacks will more often than not frustrate recipients enough for them to report the sender as spam).

For all senders, though, it means staying vigilant and agile, putting in the time and patience to listen to the data and fine-tuning email content until it clearly meets the needs and desires of its recipients.

“Email,” Meyer said, “is not a set-and-forget type of discipline.”

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