EmailEngine is a self-hosted application that opens IMAP and SMTP connections against registered email accounts to translate API requests to IMAP commands, send emails, and monitor changes on these accounts. It sends a webhook for every update on the account.

The free "Email Sending Guide for SaaS Builders" is a detailed resource for SaaS developers. It categorizes emails into three types: Transactional, Marketing, and Sending on Behalf of the User.