SMTP (Simple Mail Transfer Protocol) is a standard internet protocol for e-mail communications

SMTP is used for the data transmission from local user's machine to the mail server of the recipients.

This standard has been enforced in year 1982. In most cases communication goes via the port 25. In year 2008 SMTP standard has been improved and expanded. It is known under the name ESMTP (Extended SMTP) and standard communication port became 587.

Besides, using SSL (Secure Socket Layer) SMTP protocol can transmit the data securely (in the encrypted data stream), it is called SMTPS (Secure SMTP). SMTP is used for the data transmission from local user's machine to the mail server of the recipient.

To download email messages from the recipient's mail server locally, other protocols have been used: POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol).