SMTP (Simple Mail Transfer Protocol)
Simple Mail Transfer Protocol (SMTP) is an Internet standard for
electronic mail (e-mail) transmission. First defined by RFC 821 in 1982,
it was last updated in 2008 with the Extended SMTP additions by RFC 5321
- which is the protocol in widespread use today.
SMTP by default uses TCP port 25. The protocol for mail submission is
the same, but using port 587, and SMTP connections secured by SSL, known
as SMTPS, default to port 465.
While electronic mail servers and other mail transfer agents use SMTP to
send and receive mail messages, user-level client mail applications
typically use SMTP only for sending messages to a mail server for
relaying. For receiving messages, client applications usually use either
the POP3 or the IMAP.
While proprietary system, such as Microsoft Exchange or Lotus
Notes/Domino, use their own non-standard protocols to access their mail
box accounts on a mail server, as do webmail systems such as Hotmail,
Gmail and Yahoo! Mail; all use SMTP when sending or receiving email from
outside their own systems.
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