Simple Mail Transfer Protocol (SMTP)

Overview

When the original specification for Simple Mail Transfer Protocol, or SMTP, was published almost 40 years ago together with IMF, email as we know it today was born.

Specifications

RFC821

RFC821 ("Simple Mail Transfer Protocol") was the original SMTP specification published in 1982. It was obsoleted by RFC2821 in 2001.

RFC2821

RFC2821 ("Simple Mail Transfer Protocol") replaced RFC821. It was itself replaced by [RFC5321] in 2008.

RFC 5321

RFC5321 is the currently used standard for "Simple Mail Transfer Protocol", or "SMTP". Published in 2008, it is "a specification of the basic protocol for Internet electronic mail transport". If you are interested in all the gory details, we recommend that you read this document.

This specification has many dependencies with IMF, MIME, and other technical concepts, which can quickly become utterly confusing. Here, we provide a very short and simplified description of those portions of the specification that we felt were interesting enough to repeat here.

As the spec mentions, "SMTP transports a mail object", a mail object being described as an object that contains both an envelope and content. An SMTP client connects to a server and communicates via a session. Both client and server provide a mail transport service, and are therefore act as "Mail Transfer Agents", or "MTAs". A mail originates and terminates with a "Mail User Agent" ("MUA"). On the originating side, a MUA may, for instance, collect mail to be transmitted by a user and hand it off to an MTA. On the terminating side, an MTA would hand a mail off to an MUA.

"SMTP sessions are stateful, with both parties carefully maintaining a common view of the current state." The session is initiated by the client, which establishes a two-way channel to an SMTP server. The session must either close successful (or with a failure message), else delivery is considered to have failed.

Related Concepts

SMTP relates to:

  • Email messages, in that the SMTP protocol exists to transfer messages from one host to another

  • MTA, in that the very nature of SMTP is about specifying how a message gets transferred from one MTA to another

Try It!

You can try speaking SMTP with your demo server by connecting to it using telnet. It would not work on a real world server as it would requires encrypted traffic. See "using openssl to talk to your server"

Requirements:

  • Run the long demo (without stopping)

  • Telnet

Approximate duration: 5 minutes

Note: extra lines are inserted below to help show you what you need to type.

$ telnet 127.0.0.1 25 (1)

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 JAMES Linagora's SMTP awesome Server

ehlo test (2)

250-9a2fe463e592 Hello test [172.17.0.1])
250-PIPELINING
250-ENHANCEDSTATUSCODES
250 8BITMIME

mail from: <user02@james.local> (3)

250 2.1.0 Sender <user02@james.local> OK

rcpt to: <user01@james.local> (4)

250 2.1.5 Recipient <user01@james.local> OK

data (5)

354 Ok Send data ending with <CRLF>.<CRLF>

Subject: Test Message (5)
(6)
This is just a test. (7)
. (8)

250 2.6.0 Message received

quit (9)
1 Connect to the James server using the SMTP protocol via telnet.
2 Initiate the SMTP session.
3 Indicate that you want to send a mail from user02@james.local.
4 Indicate that the mail should be sent to user01@james.local.
5 Enter the subject.
6 Enter a blank line.
7 Enter some text. You can enter multiples lines if you like.
8 When you have completed the message, enter a line with only a "." and no other text.
9 End the session.

If you set up Thunderbird to connect to the server, you can confirm that the mail has indeed arrived in the mailbox of user01@james.local.

Pretty cool! Good thing we have James so we don’t have to always send emails manually.