Email deliverability is an incredibly complex topic. Many email marketers work in the industry for years without truly understanding how email deliverability works. That being said, email deliverability is extremely important. Why? Because no matter how many email addresses you collect and how many emails you send, it’s all worthless if your emails don’t land in the inbox.

Before going any further, let’s clarify some of the lingo. Having a good email deliverability is synonym with your emails not going into your recipients’ spam folder. The percentage of your emails that land in the spam folder is often called the bulk rate or spam rate.

Quick aside: Spam rate can also refer to the percentage of emails that are “marked as spam” for any given campaign.

Another important term to define is ISP, which technically stands for Internet Service Provider but, in the email world, ISP refers to the email service your subscribers use. These include Gmail, AOL, Hotmail, etc. On a somewhat related note, here’s an interesting blog post that ties ISP data to demographics data.

The basis for good email deliverability is sender reputation. ISPs need to know if you are someone that sends good, non-spammy, emails or not. This is extremely important since 50% of the emails sent are spam. The good news is that this number has been decreasing over the years—in 2009, 97% of emails were spam. Just imagine all the work ISPs have done to detect who’s a spammer and who isn’t!

By now you should be wondering how the heck an ISP determines if someone is a good sender or not. At a fundamental level, each ISP creates a reputation score for each sender. The exact scoring formula is unknown to prevent spammers from gaming the system. That being said, a few known variables are extremely important. These are IP address reputation and user behavior. Let’s dive into each of these next.

IP address reputation

The reason an IP address is important is because it’s the address used to send your emails. Just like you use the same phone number to dial people from your mobile device, you use the same IP address to send your emails. Defining what’s an IP address and how it works falls beyond the scope of this book, but you can read more about it here.

Essentially, you can think of your IP address reputation as the equivalent of your email program’s credit score—is this IP address reputable or not? A good IP reputation means you are trustworthy and your emails are not as likely to end in the spam folder. The opposite is true when your IP address has a bad reputation—a bad IP reputation is terrible news for your deliverability.

Just like your credit score, there are things you can do to improve (or hurt) your IP reputation. As an individual, paying your credit card bills, students loans, and mortgage on time contribute to having a good credit score. The opposite is true if you’re always late with payments. A similar dynamic applies to the email world, and this comes in the form of user behavior.

The way subscribers behave when they receive your emails rolls up and creates your IP address reputation. The more subscribers engage with your email, the better your reputation. Let’s explore this next.

User behavior

One of the biggest contributors to your IP reputation is your email open rate. On average, transactional emails receive an open rate of 90%+. But, if your other emails (such as newsletters) receive a low open rate—typically lower than 10%—that spells trouble.

This is really important… let’s repeat it: if your email campaigns open rate falls below 10%, you are in trouble. This is when ISPs such as Gmail will start dropping you in the spam folder.

Another very important metric is spam rate. If the emails you send receive a high spam rate your IP reputation takes a hit. Always monitor your spam rate and make it easy for subscribers to unsubscribe from your emails—this will decrease the likelihood of being marked as spam.

Email frequency is also very important. If you only email once a year, subscribers will have no idea who you are and are likely to mark you as spam. Now, if you email 18 times a day subscribers will get frustrated and might mark you as spam too. You need to find the right balance.

There’s a few other email engagement that affect your IP reputation. The ones that positively affect your reputation are: [users]

  • Opening and reading through the email
  • Clicking through links
  • Enabling images
  • Adding you to their address book

The ones that negatively affect your reputation are: [users]

  • Marking your email as spam
  • Deleting your email
  • Ignoring your email

As mentioned earlier, the way subscribers behave when they receive your emails rolls up and creates your IP address reputation. The more subscribers engage with your email, the better your reputation. The less they engage (or the more they ignore you / mark you as spam), the worst your reputation.

We’ve seen that your IP address plays a critical role in email deliverability. Let’s now talk about the different IP address setups you might encounter.

Different IP address setups

As mentioned earlier, an email ends up in the spam folder or in the inbox based on the IP address sending the email and this IP’s reputation.

Your IP setup will typically fall into one of three buckets: a shared IP pool, one dedicated IP, and multiple dedicated IPs.

Shared IP pool

A shared IP pool is what you get when you sign up with most ESPs. Essentially, the ESP has a “pool” of IP addresses they use to send emails on behalf of their clients (including you). The ESP takes care of managing the IP addresses and, for the most part, they worry about making sure your emails get delivered. This is why most marketers don’t know much about email deliverability—they’ve never really had to deal with it.

You might be wondering when it’s worth going from an IP address pool to your own dedicated IP address. Mailchimp has a really good blog post on this exact topic. As a rule of thumb, if you have less than 100,000 subscribers, don’t even consider researching this. If you have more than 100,000 subscribers, it’s worth spending some time looking into this.

One dedicated IP address

When your email program is essential to your company’s profitability, it often makes sense to invest the necessary time and resources to manage your own IP address.

Having a dedicated IP address gives you full control over the emails sent from this IP address. Whereas when you use a shared IP pool you don’t know what emails other companies are sending. With a dedicated IP address you can also monitor your email deliverability using tools such as ReturnPath.

Now, if your email volume is high, you may need more than one dedicated IP address. This is because ISPs limit how many emails they’re willing to receive from any given IP at a given time. As a rule of thumb, you’ll want to own one IP for every two to three million subscribers you have.

It is also recommended you use different IP addresses for transactional emails v.s. promotional emails. Why? Because you can’t afford to risk having your transactional emails go into spam—imagine your customers not receiving a purchase receipt or a shipping confirmation email. It’d be chaos for your support team. This brings us to the last IP address setup.

Multiple dedicated IP addresses

The setup is typical amongst companies with large email lists. Depending on your email volume, you may want to get an IP address for every few million subscribers. For example, if you have 6 million subscribers you’d want 4 IP addresses. 3 to send newsletters and promotions (1 for every 2 million subscribers) and 1 for your transactional emails. This is a general rule of thumb and different email marketers will have different opinions.

Another popular reason to have multiple IP addresses within one company is when your company has many brands (e.g. Ann Taylor and LOFT). You may want to make sure each brand has their own IP address so that the activity of one brand has zero impact on the other.

Now, before you can use an IP address to send emails, you need to do something that’s called “warming up the IP address”. Warming up an IP means you gradually send emails from this new IP address to show the ISPs that you are a trustworthy new send. You can’t go from zero to millions of emails at once—the ISPs would ban you. You need to ramp things up slowly, which we’ll cover next.

Quick aside: When using a shared IP pool the ESPs take care of warming up the IPs so you—and other customers—can send emails without having to worry about this.

Warming up a new IP address

Let’s say you just got a brand new IP address, now what? You need to start sending emails from it, slowly, to “warm it up”.

Quick aside: You typically take the same steps to warm up a new IP address as you would to fix an IP address that has a bad reputation.

The first step is to get the IP address and not send emails from it for a few days. Then, slowly start sending emails from it, gradually increasing volume over time. This process can take weeks depending on your list size.

An example schedule might look like this:

  • Day 1: Get the IP address
  • Day 5: Send 10,000 emails
  • Day 7: Add 20,000 emails (total send: 30,000)
  • Day 9: Add 50,000 emails (total send: 80,000)
  • Day 11: Add 100,000 emails (total send: 180,000)
  • Day 13: Add 120,000 emails (total send: 300,000)
  • Etc.

You need to closely monitor your email performance as you scale email volume.

The best way to know if your warm up strategy is working is to talk with your ESP directly and to use a service such as ReturnPath. They’re pretty much the leaders in the email deliverability monitoring space (what a mouthful!).

Whether you’re warming up a new IP address or simply sending a newsletter from an existing IP address, it’s important you email a list that’s clean. The first step to ensuring you have a clean list is to only email engaged subscribers. As a rule of thumb, if someone hasn’t opened or clicked on your emails in a year, you should remove them from your list.

This last point illustrates the importance of emailing somewhat frequently. Example: If you only send one email per quarter then you only get 3 (max 4) emails before you’d have to remove someone from your list—assuming you follow the above mentioned guideline of only emailing subscribers that opened in the last 12 months.

Removing inactive subscribers helps you keep a list that’s engaged. This is actually something the ISPs have pushed marketers to do by implementing something called spam traps—we’ll cover this in more depth in the next section.

Quick aside: Companies often send “we miss you” emails and other winback or re-activation emails as a last chance to get their emails opened. This is to prevent them from having to remove inactive subscribers from their lists.

List scrubbing best practices: Aka, how to keep a clean list

The best way to keep a list clean is to make sure subscribers are not marking you as spam. How do you do this? Well, don’t email too frequently. Make it easy for people to unsubscribe. And don’t send irrelevant content (e.g. sending an email promoting a new store opening in NYC to someone in Utah is bad).

Also, don’t ever purchase email lists or scrape websites to gather email addresses. ISPs will find you. Your IP will get banned. Your emails won’t make it to the inbox. And your boss won’t be happy.

Keep a close eye on your bounce rate (a bounce simply means that the email you’re trying to send was not able to be delivered). There are two types of bounces: soft, and hard. A soft bounce means the email address exists but the email was not delivered (e.g. the inbox is full). A hard bounce means the address doesn’t exist or is invalid.

You should never email an address that has given you a hard bounce in the past — as a matter of fact, your ESP won’t allow you to do this by default. Also, three soft bounces typically equal a hard bounce. Keep that in mind and stay on top of your bounces. My preference is to stop emailing addresses that have bounced, even if it was a soft bounce. My deliverability is more important that these few extra subscribers. If you’re interested in learning more about soft and hard bounces, checkout this article from SendGrid.

Earlier we mentioned something called a spam trap. This, my friend, is your worst nightmare. A spam trap is an email address used to expose senders that send unsolicited emails (aka: spam) or those that don’t have good list cleaning practices in place.

There’s two types of spam traps: a recycled spam trap and pure spam trap.

A recycled spam trap happens when an ISP “recycles” an email address that has gone dormant (e.g. the owner of the inbox has not opened their inbox in 7+ months). They’ll keep the email address “active” and monitor who still sends emails to this address though the address has gone dormant (as seen by a lack of opens or clicks). This is why you should not email subscribers that haven’t opened your email in the last 12 months.

Pure spam traps are emails addresses left around the web as baits. These are email addresses that have never signed up for anything or opted in to receive any emails. They are just left in the wild for people to scrape and email them—which is clearly a sign that you’re not keeping your list clean.

The reason buying an email list can be really bad is that you run into the risk of hitting both types of spam traps. If you’re interested in learning more about spam traps, checkout this article from Campaign Monitor.