SMTP. Example of the session

SMTP protocol is based on simple text-like commands.

Client communicates with a server via these text commands. The sender as well as server-transmitter can initialize the session.

When connection is established (via TCP usually) new session is opened. Three main commands have been used:

Dos session > telnet smtp.server.com 25
S: 220 smtp.server.com Welcome to smtp.server.com
C: helo Alex
S: 250 smtp.server.com
C: MAIL TO:<sender@domain.com>
S: 250 Ok
C: RCPT TO: <receiver1@domain.com>
S: 250 Ok
C: RCPT TO: <receiver2@domain.com>
S: 250 Ok
C: DATA
S: 354 End data with
C: Subject: Test message
C: Hello everyone
C: This is a test message
C: Best regards
C:.
S: 250 Ok queued as 12AB34CD56
C: QUIT
S: 221 Bye

After the session is ended server disconnects.

SMTP server responds to greeting command (helo) and, if session started from this greeting, server enforces SMTP protocol.

All servers have limitation on size of the message being transmitted via SMTP protocol. While transmitting via SMTP, sender has no idea of server's limitations, and negative response can be obtained only after when transmission has been completed (it can be very large volume of data and it can take a lot of time and resources). That's why SMTP protocol was extended to ESMTP.

ESMTP protocol starts session from another greeting (ehlo). All modern Mail User Agents (MUI) start communications from ehlo and only after obtaining negative response (502: Error: command not recognized) they try to send standard greeting helo.

Example:

Dos session > telnet smtp.server.com 25
S: 220 smtp.server.com Welcome to smtp.server.com
C: ehlo Alex
S: 250 smtp.server.com
C: 250-SIZE 123456
S: 250-PIPELINING
C: 250 HELP
...