4. MustachioLightweight, powerful, flavorful, template engine.
6. PactmakerStarter workflow for creating electronically signed PDF agreements.
11. MailmasonA complete toolset to streamline building and updating a set of consistent transactional emails.
12. Postmark RailsOfficial integration library for using Rails and ActionMailer with the Postmark HTTP API
15. mailhandlerSimple lightweight mail library which allows you to send and retrieve emails, and get more details about email sending and delivery
17. smtp-field-manualA collection of raw STMP server responses from major email service providers and spam filter services.