An Introduction To Email Deliverability

What is deliverability, and why is it important?

Deliverability measures how many of your emails are going to Subscribers’ inboxes vs. their spam folders. After putting so much work into your email marketing, you want to be sure that your Subscribers are actually seeing your messages! This is why deliverability is crucial. 

How is deliverability determined?

There are a lot of factors that influence inbox placement. In general, mailbox providers are trying to determine three things:

  1. Is the message safe?
  2. Do most Subscribers want the message?
  3. Do you think this particular Subscriber would like the message?

If the answer to all three questions is yes, the message belongs in the inbox. If any of the answers above are no, this message belongs in the spam folder. Mailbox providers typically turn to the sender reputation they have calculated for you to make this decision. 

Sender Reputation

Mailbox providers, such as Gmail, Microsoft, Yahoo, etc., create a reputation score for your sending domain. You can think of this like a credit score, but unfortunately, no one source of truth can indicate exactly what your score is and why. Mailbox providers keep your score and the algorithms used to determine it private. However, the most important determining factor is how subscribers engage (or don’t engage) with your message.

Subscriber Engagement

The best way for mailbox providers to determine where your mail belongs is by seeing how your Subscribers interact with it. There are actions your Subscribers can take that will raise your sender’s reputation and actions that will lower your sender’s reputation. 

Here are some recommendations for promoting positive engagement:

List Health

  • Only import Subscribers who have given you direct permission to receive email marketing. Quality is much more important than quantity when it comes to email marketing. You don’t want anyone on your list who doesn’t want to be there. For the best deliverability, be sure everyone on your list knows precisely what they are signing up for.
  • Secure your Forms with a double opt-in and/or ReCaptcha. If your Form isn’t secure, there’s a good chance you’ll start to see bots abusing it. We talk more about why this is harmful to your deliverability here.
  • Prune your list regularly. Be sure you don’t let Cold Subscribers pile up. If you don’t clean your list regularly, the unengaged Subscribers will start to become the majority. When this happens, your sender’s reputation drops, and more messages start to be filtered to spam. Learn more about how to prune your list here.


An important component of your sender’s reputation is consistency. If there is any sudden change to how you send messages, mailbox providers will see your messages as risky and potentially place them in the spam folder. Here are some characteristics you’ll want to keep consistent:

  • Sending Domain: It’s perfectly fine to send from multiple domains, but be sure the domain you’re sending from is the domain that Subscribers will recognize. Avoid sending from a brand new domain or a domain that has not sent any mail in a long time (over six months). 
  • Volume: Be sure to send to a consistent number of Subscribers each week. It’s normal for your list to shrink and grow, but try not to have any significant volume swings. (You don’t want to jump from 5,000 Subscribers to 20,000 Subscribers overnight!) If you need to introduce a large number of new Subscribers, be sure to start sending to them in small batches so you don’t alarm mailbox providers.
  • Frequency: Send at least once per month to keep your sender reputation established. If mailbox providers don’t see enough mail from you, they won’t be able to maintain a reputation for you, and you’ll be starting from scratch on your next send. Also, be sure not to send too frequently to Subscribers — it could cause them to mark your messages as spam! We recommend asking your Subscribers ahead of time how often they’d like to hear from you and allowing them to modify their preferences in each email.


While content matters much less than it used to when it comes to inbox placement, it still has the potential to influence your deliverability – especially if your sender reputation isn’t well established. 

If mailbox providers haven’t seen consistency from you, as we covered above, they might not have enough information to decide whether your message is safe and wanted. This means they might need to rely on your content as another data point to make this decision.

Conversely, if mailbox providers have seen enough consistent volume from you that they have confidence in how you’ve been scored, they’re more likely to ignore your content totally. Here are some things to keep in mind:

  • Don’t Use Link Shorteners: Because link shorteners are used so often by spammers, they can raise a red flag for spam filters. Transparency is key for good deliverability. When adding a link to your emails, link directly to the website you’re sending subscribers. 
  • Spammy words and phrases: Marketers’ most significant mistake is using overly promotional and/or urgent language. Even if you are selling something in your email, try to write your email so that it sounds human and conversational. 
  • Image-to-text ratio: There’s no magic number here, but the best guideline to follow is to have enough text in your email so that if images fail to load, you’d still have a great email that communicates your message.
  • Avoid gimmicks: While you might see a quick boost in open rates from a subject line that says “Re: Your Order #2095642”, you’ll lose Subscribers’ trust. Your audience will likely mark the message as spam or disengage with your emails. Avoid quick tricks to boost engagement.
  • Encourage authentic engagement: Give your Subscribers permission to engage with you authentically. This engagement will help boost your sender reputation and also help build a relationship with your Subscribers. A fun example would be asking your Subscribers to reply to your message with their favorite song right now. 


Influence mailbox providers to place your messages in the inbox by becoming Subscriber-focused. Give your subscribers the power to choose the type of content they want to receive and how often they want to receive it. Build a list of Subscribers who want to be there, and create the kind of content that keeps them engaged!