SMTP is the Simple Mail Transfer Protocol, the most recent standard of which is defined in RFC 5321. It's the Internet standard for sending and receiving email.
Some characteristics of SMTP:
- The default port is TCP port 25 and new submissions (MSA) is port 587.
- Although depreciated, some services still use port 465 (SMTPS) to support legacy applications.
- It is used to transmit outbound mail, and mail servers (which listens on port 25) receives mail on port 25.
- It should not be confused with POP3 / IMAP which is an actual store for the e-mail. SMTP messages are received by the receiving server and then an MTA moves it to the POP3 / IMAP mailbox where it is normally retrieved using a POP3 / IMAP client.