Postal vs Postmark: What are the differences?
Developers describe Postal as "Open source mail delivery platform, alternative to Mailgun or Sendgrid". Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers. Postal was developed by aTech Media to serve its own mail processing requirements and we have since decided that it should be released as an open source project for the community. On the other hand, Postmark is detailed as "Email delivery for web apps - done right". Postmark removes the headaches of delivering and parsing transactional email for webapps with minimal setup time and zero maintenance.
Postal and Postmark can be categorized as "Transactional Email" tools.
Postal is an open source tool with 9.02K GitHub stars and 533 GitHub forks. Here's a link to Postal's open source repository on GitHub.