First make sure you follow all the steps in Improve Your Email Deliverability. Once you have taken care of these steps you should follow these best practices in order to avoid SPAM folders:
- Make sure your IP is not on any black/grey lists. You can test it here mail-tester.com, and you should aim a score above 7.0/10.0.
- Reduce image to text ratio (big images are bad)
- Reduce spammy words (e.g. $, big numbers, free, sale, etc.)
- Personalize the "to" field with recipients name
- Avoid having too many of your emails bouncing.
- Run a delivery doctor or inbox inspector before delivering an email
You should also make sure that you follow our Email Best Practices and Writing Resources in order to create personalized emails that will not get marked as SPAM.