Email deliverability is a complex topic but there are simple things you can do to make sure your emails don't get caught by spam filters.
Set up domain authentication (DKIM, SPF) and custom tracking domain. This is technical and 100% unexciting but the good news is it's binary: once you do this you'll start seeing results right away. Read our detailed instructions for authenticating your domain and setting up custom domain tracking.
Avoid words that trigger spam filters in your subject line and body copy. Spammers have "trained" inbox filters to recognize certain words as strong signals for spam. Sometimes you can recognize these words from a mile away (eg. viagra or weight loss plan) but some spam trigger words look perfectly legitimate (eg. email marketing or laser printer). Consult this article to know which words to avoid in your emails. Make sure to also avoid shortened URLs (like bit.ly for example) as they can trigger spam filters. Links in the email content should be visible and easy to understand so recipients know where they go when they click them.
Structure your emails to look like ... normal emails. Spam filters like emails that look as if someone emailed their friend. This means that the email is not super-designed and doesn't contain a lot of html-code. This also means that text-to-image ratio is 80:20 ie. if it does contain an image there's also a good amount of accompanying text. Bonus points for keeping the number of links small (1-3 links is better than 5-10 links).
If you're using a brand new domain, warm it up first. Start small and gradually increase the volume of emails you send (for example 50 emails per day on the first week, 100 emails per day on the second week, etc.). You'll also want to generate some incoming email traffic. For example, sign up for some high-quality newsletters (think NYTimes, or why not our email course) and ask people to reply to your emails.
Clean your lists. Invalid and stagnant emails in your list lower your deliverability rate and might trigger email providers to mark your emails as spam. We recommend using tools like MyEmailVerifier to clean your lists every 6 months.
There are many more things you can do to improve your email deliverability. You can read our full email deliverability guide here.