Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is an application-layer protocol for sending email from clients to mail servers and between mail servers. It uses commands and status codes to conduct a message-transfer session.

SMTP command: A text instruction sent by an SMTP client to control a mail-transfer session. Commands such as EHLO, MAIL FROM, and RCPT TO structure SMTP conversations.
Transmission Control Protocol: A transport protocol that provides reliable, ordered byte streams between networked applications. SMTP commonly carries its sessions over TCP connections.
Email submission: The process by which an email client hands an outgoing message to a mail server for delivery. SMTP submission commonly uses authenticated connections on ports 587 or 465.
Post Office Protocol: A protocol that lets an email client retrieve messages from a mailbox server. POP retrieves stored messages, while SMTP sends and relays them.
SMTP reply code: A three-digit status code returned by an SMTP server to describe the result of a command. Reply codes report whether each step succeeded, failed, or needs further action.
Internet Protocol: The network-layer protocol that routes datagrams between hosts using IP addresses. IP routing delivers the TCP connections on which SMTP operates.
Message transfer agent: Software that routes email between mail systems using store-and-forward delivery. MTAs use SMTP to relay messages toward their destination domains.
Internet Message Access Protocol: A protocol that lets clients access and manage messages stored on a mail server. IMAP handles mailbox access rather than SMTP’s message transmission.
ESMTP: An extension framework that adds capabilities to the original SMTP protocol through advertised features. Modern SMTP sessions commonly use ESMTP to negotiate supported capabilities.
Domain Name System: A distributed naming system that maps domain names to records used by internet services. SMTP uses DNS mail-exchanger records to find a domain’s receiving servers.

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