4 Things You Might Not Know About Email Deliverability

Every day, ISPs are surveying the quality of the emails you send based on countless signals - they take everything (but the kitchen sink) into consideration when deciding where your future campaigns will end up.

We put together this list of email deliverability fundamentals to shine a light on some of the things you might not know about landing in the inbox. Hopefully you’ll learn a thing or two, or be reminded of something you haven’t recently considered!

1. Deliverability varies based on individual subscriber engagement.

Think of how you use your personal inbox every day (whether it’s Gmail, Hotmail, or another ISP). You’re reading emails, clicking links, archiving things you don’t care about, and marking the odd something you definitely don’t remember subscribing to as spam.

What you might not be thinking about is all the signals you’re sending to your ISP. As you go through your inbox every day, your ISP takes note of what you open, what you don’t, and what you click on. They use these signals to dictate what will get top inbox placement for you in the future - maybe you’ve seen the newsletter you engage with every day move from Promotions to Primary (see theSkimm below), or maybe you’ve noticed those marketing updates from a brand you haven’t bought from in over a year in your spam folder.


If you start thinking of all your subscribers in this light, you’ll get a better idea of what you need to do to improve deliverability. Actively thinking about each of your subscribers’ inboxes incentivizes creating emails that you know your subscribers will engage with, and disincentivizes emailing inactive or poorly targeted contacts.

2. ISPs check the content of your email before assigning it to inbox or spam.

Think of ISPs like computers that can read, but can’t see. Consider this while designing your next campaign to give them more context as to why your email deserves inbox placement.

Things to consider:

  • ALT text. Though your ESP can’t see what images you’re using, they’ll read the ALT text to decide how relevant your email is (see Complex’s Welcome Email below).
  • Body text. Emails with just an image and a button don’t provide a lot of context - ISPs use body text to assign relevance. Depending on the kinds of emails you’re sending, you may need to play around with text-to-image ratio - expert opinions vary from 80:20 to 60:40.
  • Links. Some link shorteners will automatically get your email marked as spam, so make sure you’re using a reputable one. Triple-checking the links you’re adding to emails is also important - broken links decrease deliverability.


3. Sending to inactive subscribers can hurt deliverability to engaged subscribers.

Every time you send an email, your sending reputation is affected. The good news is, you have the power to control whether your campaigns positively or negatively affect your deliverability.

A surefire way to negatively affect the deliverability of future campaigns is to consistently send emails to inactive subscribers. This sends a signal to ISPs that not only do your inactive subscribers still not want to receive your emails, but that your emails might also be irrelevant to subscribers who have engaged in the past. The more you do this, the more likely ISPs are to flag your campaigns as “similar to messages that were detected by our spam filters.”


Sending irrelevant messages to inactive subscribers will hurt your deliverability. Simply put: don’t do it!

(Worth nothing: there are ways to intentionally re-engage inactive subscribers and get them back into the buying cycle. Check out this post on creating a winback campaign: Email Automation 101: How to Create an Engaging Winback Series.)

4. You should use subdomains for different types of emails to ensure good deliverability.

It’s not uncommon for email marketers to use one domain for all email marketing purposes - this is a pretty big no-no.

Subdomains let you separate out your sending reputation for different types of emails.


Separating your reputation is especially important for brands sending lots of email, which often gets split into transactional (think: order receipts, shipping notifications, password resets) and marketing emails (promotions, new releases, etc.).

These kinds of emails are totally different in the purposes they serve, which is why you shouldn’t let the sending reputation of one affect the other. The most likely scenario is that the sending reputation of your marketing emails will bring down the deliverability of your transactional emails.

The key takeaway here is to make sure you’re using a subdomain vs. your main domain for marketing emails, and another subdomain for transactional emails. For a deep dive into this topic, check out this post: Why You Should Be Using A Subdomain.

Be mindful of deliverability

We think it’s vital to keep your sending reputation top-of-mind when doing anything in the realm of email marketing - though it can be a little intimidating, once you wrap your head around a few key concepts that affect deliverability, you’ll be well on your way to building a strong sending reputation that sustains over time!