SMTP
Simple Mail Transfer Protocol — the standard protocol used to send and relay email messages between mail servers. It is used for outgoing mail only; reading email requires IMAP or POP3.
SMTP (Simple Mail Transfer Protocol, RFC 5321) is the protocol that mail servers and email clients use to transmit outgoing messages. When you click Send in an email client, the client connects to an SMTP server, authenticates, and hands off the message for delivery. The SMTP server then relays the message from server to server until it reaches the recipient's mail server.
SMTP handles only the transmission of messages, not their storage or retrieval. Once a message arrives at the destination server, protocols like IMAP or POP3 allow mail clients to read it. This separation of concerns means that SMTP is not involved in reading, archiving, or searching email — those tasks fall to other protocols and formats.
SMTP servers add Received headers to messages as they pass through the relay chain. These headers, visible in the raw message source, record the IP addresses and timestamps of each server that handled the message, which can be useful for tracing delivery issues or verifying authenticity.
Related terms
Internet Message Access Protocol — the standard protocol for accessing email stored on a server, keeping messages synchronized across multiple devices without downloading and deleting them.
Post Office Protocol 3 — an older email retrieval protocol that downloads messages from a server to a local device, typically removing them from the server afterward.