Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is a communications protocol used to transfer electronic mail messages efficiently from one server to another.

Simple Mail Transfer Protocol is only used for server-to-server communication. Simple Mail Transfer Protocol servers route the messages through the Internet to a mail server, usually a Post Office Protocol account, which then holds the messages for future retrieval by the user.

Simple Mail Transfer Protocol presumes that the Transmission Control Protocol/Internet Protocol is the primary protocol.