Simple Mail Transfer Protocol

SMTP, or Simple Mail Transfer Protocol, is a protocol for sending e-mail messages between servers.
Most E-Mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a Mail Client to a Mail Server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.