Sendmail is the probably the most widely-used UNIX Internet mail package. Other UNIX Internet mail packages are often designed to operate much like sendmail. Sendmail applies a bizarre series of transformations to mail, which I won't dare to touch on here. Suffice it to say that sendmail runs in two major modes. One is as a daemon, the program being detached into background during a startup script, listening for incoming SMTP mail, acting as an SMTP server, and delivering that mail. The other mode is invoked from a mail program or even the command line, sendmail accepting RFC 822 mail over standard input. A caveat here - the invoking user or program has to parse the destination addresses and present them on the command line.