Ask HN: How to send per-customer emails by timeline?

Looking for a way to send per-customer e-mails according to a release timeline. Is there a platform that has this functionality?

Example: New Signup gets Welcome email, gets Week 1 email 7 days later, Week 2 email 7 days after that.

Mass e-mail by hand is getting unwieldy. I am not super familiar with solutions in the space and would appreciate any helpful pointers. Could optionally roll-my-own but that's time detracting from the main product.