What is SMTP?

Simple Mail Transfer Protocol (SMTP) is the standard application-layer protocol for transmitting electronic mail. Its primary role is to facilitate the relay of outgoing mail from an email client to a mail server and subsequently from that sending server to the recipient’s mail server.

When an application sends an email, it connects to an SMTP server, often authenticating with credentials. The server then processes the message, determining the recipient’s domain and routing it through a series of Mail Transfer Agents (MTAs) until it reaches its final destination.

It is important to distinguish SMTP’s function from retrieval protocols. SMTP is a push protocol used exclusively for sending messages. The corresponding pull protocols, such as IMAP and POP3, are used by clients to retrieve messages from a mail server.