IF01083

SMTP

SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy to test an SMTP server using the telnet program (see below).

SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is typically used, falling back to a simple A record in the case of no MX (not all MTAs (Mail Transfer Agents) support fallback). Some current mail transfer agents will also use SRV records, a more general form of MX, though these are not widely adopted.

SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN.

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