Some characteristics of SMTP:
- The default port is TCP port 25 and new submissions (MSA) is port 587.
- 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.