Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending email between:

SMTP communications typically occur on TCP port number 25.

When an email user sends an email, their MUA uses SMTP to send the email to an MTA, which is often their email server. The MTA then uses SMTP to directly or indirectly deliver the email to the destination email server that hosts email for the recipient email user.

When an MTA connects to the destination email server, it determines whether the recipient exists on the destination email server. If the recipient email address is legitimate, then the MTA delivers the email to the email server, from which email users can then use a protocol such as POP3 or IMAP to retrieve the email. If the recipient email address does not exist, the MTA typically sends a separate email message to the sender, notifying them of delivery failure.

While the basic protocol of SMTP is simple, many SMTP servers support a number of protocol extensions for features such as authentication, encryption, multipart messages and attachments, and may be referred to as extended SMTP (ESMTP) servers.

FortiMail units can scan SMTP traffic for spam and viruses, and support several SMTP extensions.