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Postfix is Wietse Venema's mailer that started life as an alternative to the widely-used Sendmail program. Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.
Postmaster implements an ESMTP server. Given a configuration, it starts up and listens for incoming SMTP connections, handles them, and pipes the accepted e-mail messages into an arbitrary local mailer of your choice. A good local mailer is Procmail.
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Beyond that, you can configure and modify every little step of the SMTP transaction. All the real work is done through call-back functions. [Less]
Mailster is a project aimed at testing software mail capabilities. It provides a mail server container to test emails sent by your apps without rewriting your application code.
With support for all the basic SMTP commands, php-smtp is an easily deployed and managed SMTP server built in PHP.