Simple Mail Transfer Protocol. The most common protocol used for transferring email across the Internet.

Most email 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 email client using either POP3 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 POP3 or IMAP server and the SMTP server when you configure your email application.

SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN.

SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is used.