Simple Mail Transfer Protocol

SMTP is one of several application layer protocols dedicated to e-mail. As with other protocols, SMTP describes a conversation or negotiation between two parties: a sender and a receiver. When an email is written, the first thing that happens after hitting send is that the message gets transferred from the sender's local device to their remote mail server. Then, this outgoing mail server negotiates with the recipient's incoming mail server. SMTP describes the way that the two mail servers need to interact in order to validate each other's role in the communication process.

SMTP governs the communication from a sender to their mail server, and from their mail server to that of the recipient. In other words, it is used only to send e-mail. Other protocols describe how e-mail can be retrieved from a mail server by a recipient. SMTP also tends to run on a special well-known port, port 25.

Relevant Note(s): Network Protocols