SMTP is the short form for Simple Mail Transfer Protocol. The Simple Mail Transfer Protocol is a communication protocol for electronic mail or email transmission. As an Internet standard, SMTP was first defined in 1982 by RFC 821. Later it was updated in 2008 by RFC 5321 to Extended SMTP. Extended SMTP is the protocol variety which is in widespread use today. Mail servers and other message transfer agents use SMTP to send and receive mail messages. Proprietary systems like Microsoft Exchange and IBM Notes and various web mail systems such as Outlook.com, Gmail and Yahoo! Mail may use various non-standard protocols internally, but all use SMTP when sending to or receiving email from outside their own systems. SMTP servers commonly use the Transmission Control Protocol or TCP on port number 25.
It is to be noted that, various user-level email clients typically use the SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 as per RFC 8314. For retrieving messages, the IMAP and POP3 are standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.