Postfix vs Postmark: What are the differences?
Developers describe Postfix as "A free and open-source mail transfer agent". It is a free and open-source mail transfer agent that routes and delivers electronic mail. It is Wietse Venema's mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Now at Google, Wietse continues to support it. 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.
Postfix and Postmark can be categorized as "Transactional Email" tools.