SMTP

SMTP, or Simple Mail Transfer Protocol, is a TCP-based protocol used for delivering electronic mail between mail servers. It is a core component of the Internet email architecture and handles the transfer of messages from a sending server to one or more receiving servers. SMTP is designed for server-to-server transmission; retrieval and user access are typically handled by other protocols such as POP3 or IMAP.

History and standards: SMTP was defined in its early form by RFC 821 in 1982 and later updated by RFC 5321. The format of Internet messages is specified by RFC 5322. The common usage model involves mail submission from a client to a mail server on port 587 (with authentication) and inter-server delivery on port 25. Port 465 has historically been used for SMTPS (SMTP over TLS) but is not part of the current core standard, though it remains widely supported by many servers.

How it works: A client opens a TCP connection to an SMTP server and begins with a greeting (EHLO or HELO). The client may authenticate using AUTH if required. The client then issues MAIL FROM and RCPT TO commands to specify the sender and recipients, followed by DATA to transmit the message content. The message ends with a line containing only a period. The server responds with numeric status codes indicating success or failure and then the connection is closed with QUIT.

Security and abuse: SMTP can operate over TLS via STARTTLS to encrypt a session or, less commonly, over TLS from the start. Because SMTP was designed for openness, servers commonly implement relaying restrictions and anti-spam measures such as authentication, DNS-based checks, and policy controls.