On the Internet, sendmail is the most popular UNIX-based implementation of the Simple Mail Transfer Protocol (SMTP) for transmitting e-mail. When a sendmail server receives e-mail, it attempts to deliver the mail to the intended recipient immediately and, if the recipient is not present, it queues messages for later delivery. However, because it does not provide a mailbox facility and for other reasons, other software such as a POP3 or Internet Message Access Protocol server are also needed. Most Internet service providers (ISPs) provide both an SMTP server (such as sendmail) and a POP or IMAP server.
A commercial version of sendmail, called Sendmail, includes a POP3 server and other enhancements to the basic open source sendmail. It also comes in a version that can be installed on a Windows NT operating systems platform.