SMTP (Simple Mail Transfer Protocol) is an email transmission protocol used to send email messages from a client to a mail server and to relay messages between mail servers. It is responsible for the routing and delivery of outgoing mail but does not provide message retrieval or storage functions. SMTP usually operates over TCP port 25 for server-to-server communication, or ports 587 and 465 for authenticated and encrypted client submissions. SMTP works in conjunction with retrieval protocols such as IMAP to provide complete email functionality.
