SMTP is a protocol for sending e-mail across the Internet. While any individual organization may implement a specific application for handling e-mail internally (Microsoft® Exchange, Lotus Domino®, etc.), SMTP is the common format into which all messages are converted before being sent over the Internet.

In situations where e-mail is stored on a remote server and then forwarded to the user (where a home user connects to the Internet through an ISP and downloads e-mail periodically, for example), POP3 or IMAP protocols are often used also.