SMTP

In short: Simple Mail Transfer Protocol — the standard protocol for sending emails between mail servers and from the client to the mail server.

In more detail: SMTP only handles sending/delivery, not retrieving emails — that’s what IMAP or POP3 are for. Classically runs over port 25 (server to server) or 587 with encryption (client to server).

In Depth

The path of an email typically runs over several SMTP hops: the sender’s email client delivers the message via SMTP (usually port 587, encrypted) to its own outgoing mail server. This server determines the recipient’s responsible mail server via an MX DNS record and delivers the message there via SMTP (classically port 25) — from server to server, no longer directly from the original client. Only then can the recipient retrieve the message from their mail server using a separate protocol such as IMAP.

SMTP was originally designed without authentication, which historically made it very vulnerable to spam and sender forgery — anyone could enter any address in the sender field. Modern protective measures such as SPF, DKIM and DMARC therefore supplement SMTP with additional checking mechanisms that allow receiving servers to verify whether an email was really sent by the server responsible for the stated sender domain.

See also: IMAP, Mail server