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Haraka

Haraka is an open source SMTP server written in Node.js which provides extremely high performance coupled with a flexible plugin system allowing Javascript programmers full access to change the behaviour of the server. It is used heavily in some high traffic sites - see the "Known Users" link above for testimonials.

Sendmail

Sendmail

Sendmail is a general purpose internetwork email routing facility that supports many kinds of transfer and delivery methods, i ...

Exim

Exim

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Inter ...

OpenSMTPD

OpenSMTPD

OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard exte ...