Smail

Smail-3 is a mail transfer agent (MTA) used on Unix-like operating systems. It is Free Software and is licensed under the GNU GPL but it is not a part of the GNU Project and it is not owned by the Free Software Foundation. It aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail and for routing between disparate networks. It is still in use on the Internet, but it has been mostly superseded by Exim (which was originally based on Smail) and other more modern Mail transfer agents.