SMTP protocol for sending e-mails

What is an SMTP server?

Definition of SMTP

The SMTP protocol was defined in 1982 by RFC 821. SMTP enables e-mail to be sent over the Internet, and is used to communicate on port TCP/25 between MTA (Mail Transfer Agent) services.

It is one of the oldest Internet protocols, and has always been backward-compatible. The protocol has undergone a few slight changes, with RFC 2821 and the ability to handle attachments via the MIME protocol (RFC 2045 to 2049), but the basis and its primary foundation have remained unchanged. SMTP is a fairly simple protocol, which could almost be reduced to the use of 4 commands: HELO, MAIL FROM, RCPT TO, DATA.

Glossaire SMTP


Example of an SMTP transaction with a mail server:

z03:~# telnet 25
220 ESMTP YSmtp service ready
mail from: <>
250 sender <> ok
rcpt to: <>
250 recipient <> ok
354 go ahead
Subject: Test
250 ok dirdel


ALTOSPAM is entirely based on this protocol and analyzes e-mails directly during the SMTP transaction. ALTOSPAM is a service consisting of two SMTP filtering servers.

The online tool provided on the ALTOSPAM website enables you to check DNS servers and SMTP servers for a given domain name.

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