Born in El Cerrito, California, Allman knew from an early age that he wanted to deal with computing later in life, hacking into his high school's mainframe and later using the UC Berkeley computing center for his computing needs.
In 1973, he entered UC Berkeley, just as the UNIX operating system began to become popular in academic circles. As the UNIX source code was free, the local hackers quickly made many extensions to the AT&T code.
One such extension was delivermail, which in 1981 turned into sendmail. As an MTA, it was designed to deliver e-mail over the still relatively small (as compared to today's Internet) ARPANET, that consisted of many smaller networks with vastly differently formats for e-mail headers.
Sendmail soon became an important part of BSD, the Berkeley Software Distribution and continues to be the most widely used MTA on UNIX and Linux systems today, despite its somewhat complex configuration syntax and frequent abuse by Internet telemarketing firms.