ZMailer is a SMTP mail transfer agent for Linux, BSD and other Unix-like systems.It is intended for gateways or mail servers or other large site environments that have extreme demands on the abilities of the mailer.It was first developed in 1988, in reaction to the then existing problems with Sendmail, by Rayan S. Zachariassen at the University of Toronto. The lead developer, since at least 1994, is Matti E.