Sendmail

Sendmail is the name of the most popular UNIX-based program used for the SMTP mail protocol. Sendmail is the program in charge of sending and receiving e-mail on a server. When an incoming message arrives, sendmail attempts to deliver the message immediately. If the recipient is not available at the moment, it queues the message for later delivery. Sendmail does not, however, store the queued messages, so it needs another utility, usually a POP or IMAP server. A commercial version of sendmail includes a POP3 server as well as other enhancements.