Simple Mail Transfer Protocol | RFC2821

1. A protocol for sending e-mail messages between servers.
Most mail systems that send mail over the Internet use SMTP to send
messages from one server to another.

2. SMTP is generally used to send messages from a mail client to a
mail server. This is why you need to specify both the POP3 or IMAP server and
the SMTP server when you configure your e-mail application.

See also : POP3, IMAP