There are two types of emails that I'm familiar with: marketing and transactional.
Transactional emails happen when the user takes an action (e.g. signup -> welcome email). Marketing emails happen on a regular basis (the Mon, Tues, Wed, Thurs series of emails all users get). And then there are emails that fall somewhere in between (1,2,3,4 days out, 1,2,3 weeks out, etc.).
You already have a database maintaining user information, preferences, etc, so now you need to connect that with a communication platform, and send out updated content. There are a number of platforms that will help you create these emails (mailchimp, etc.) and help you filter your audience. Typically these channels require a bit of integration, to pull preferences, habits, personal information for customization purposes.
If this sounds like the problem you're trying to solve, your best bet is to Google "mailchimp vs" and explore the options, and reviews.