sendmail
The BSD Unix Message Transfer Agent supporting mail transport via TCP/IP using SMTP. Sendmail is normally invoked in the background via a Mail User Agent such as the mail command.
Sendmail was written by Eric Allman at the University of California at Berkeley during the late 1970s. He now has his own company, Sendmail Inc.
Sendmail was one of the first programs to route messages between networks and today is still the dominant e-mail transfer software. It thrived despite the awkward ARPAnet transition between NCP to TCP protocols in the early 1980s and the adoption of the new SMTP Simple Mail Transport Protocol, all of which made the business of mail routing a complex challenge of backward and forward compatibility for several years. There are now over one million copies of Sendmail installed, representing over 75% of all Internet mail servers.
Simultaneously with the announcement of the company in November 1997, Sendmail 8.9 was launched, featuring new tools designed to limit junk e-mail. SendMail 8.9 is still distributed as source code with the rights to modify and distribute.
The command
sendmail -bv ADDRESS
can be used to learn what the local mail system thinks of ADDRESS. You can also talk to the Sendmail daemon on a remote host FOO with the command
telnet FOO 25