The SMTP Protocol


SMTP is a network protocol from the application layer that transmits mail (message) between:

The SMTP protocol works via a series of SMTP command between the server and the client.

Each command has:

The server will then respond with a reply code and a human text that defines if the command was successful.


For an example on how to transmit a message with the SMTP protocol, see this howto:

How to send an email at the command line with SMTP? Email transaction explained

Note on transmission

You can transmit (ie transport) an email:


of the SMTP client, if one is available.

  • ETRN,
  • MAIL FROM, Identifies the return address (not the sender, the sender is in the message)
  • RCPT TO, Identifies the recipients
  • VRFY, verify the username exists on the servers
  • STARTTLS is a StartTLS m to secure the connection with TLS. It takes no parameters. For PostFix, see Postfix - TLS (SSL) configuration). With mta-sts, you can define your policy publicly.
  • DATA (identified start of the message ie followed by a the body email
  • BDAT: send the data in chunk, not line by line as DATA does.
  • RSET - Reset, abort transaction and discard info
  • NOOP - No operations, only elicits an OK from the server used to check the connection.


  • SEND - send message to terminal
  • TURN

Generally forbidden (postconf.5.html)

  • GET,
  • POST


  • AUTH LOGIN (then username password)

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