What is the Simple Mail Transfer Protocol (SMTP)?
The Simple Mail Transfer Protocol, or SMTP, is the de facto standard for electronic mail transport across the Internet. When you send an e-mail message, SMTP packages your message in a kind of envelope and relays it to its destination. Multiple servers are often involved in the transport of the message, and as it passes through them, each one time-stamps and tags it. Thus, when the message arrives, the recipient can get an idea of where it’s been, and when it was sent. SMTP also handles error messages, sending notifications to senders when there is difficulty delivering their mail.
When you set up a POP mail or IMAP client, such as Outlook Express, Outlook 98 or Eudora, you must specify an SMTP server. This is for your outgoing mail and may be different from the server you use to check incoming mail.
The Simple Mail Transport Protocol was introduced in a series of RFCs, notably 772, 780, and 788, with the current base specification appearing in the 1982 RFC 821:
Work continues on SMTP, and over the years its capabilities have been significantly enhanced. Current programs that implement SMTP, such as
sendmail, are based on these Extended SMTP (ESMTP) standards. Important RFCs that cover extensions or otherwise discuss SMTP include: