ZMailer is an MTS for UNIX and UNIX-like systems that was originally written by Rayan Zachariassen and is now maintained by Matti Aarnio.
ZMailer has an official web page run by the author, with links to a mailing list that the author runs.
It supports some of the more recent innovations that one would expect of a modern mail system:
Like qmail and postfix, ZMailer comprises a suite of programs, each of which handles one area of the overall job:
ZMailer's modularity is not as fine grained as that of qmail.
Like Sendmail, ZMailer actually implements a whole language of its own for its mail routing configuration files, which an administrator has to learn. Unlike Sendmail, ZMailer at least makes some effort towards having a language that is legible by human beings. The syntax of ZMailer's routing configuration language resembles that of Bourne shell scripts.
Automated configuration of ZMailer cannot be easily done under shell script control with the likes of echo and cat.
Matti Aarnio's version of ZMailer is available by anonymous FTP. Older versions are available by FTP from the University of Toronto.