SMTP, testing via Telnet

When troubleshooting problems with SMTP service - your own, or others - it is frequently very helpful to be able to "speak" to the SMTP server directly, rather than going through a mail client which won't necessarily tell you exactly what the SMTP server is saying. You can easily do this with the telnet client.

Note that many ISPs do not allow outbound connections on port 25 to any SMTP server but their own - if you get timeouts when trying to connect to port 25, you should try port 587, which is the standard ESMTP port. (Port 587 connections normally require SMTP AUTH, which is covered below.)

Testing an SMTP service via telnet

ph34r# telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
220  ESMTP
HELO justtesting
250
MAIL FROM: me@telnettingin.com
250 ok
RCPT TO: postmaster@mail.getsdeliveredhere.net
250 ok
DATA
354 go ahead
To: postmaster@mail.getsdeliveredhere.net
From: telnetclient@mail.getsdeliveredhere.net
Subject: this is a test message
Date: Thu, 21 Jun 2007 11:11:40 -0400
Just testing SMTP functionality by telnetting in to port 25.  I'll end this message now
by entering in a line with nothing but a period in it and hitting return.
.
250 ok 1103093638 qp 87827
QUIT
221
Connection closed by foreign host.

Okay - our SMTP server just accepted a telnet connection, responded like a mailserver, and accepted a nice little test email for delivery. (Any response other than a 250 ok would represent an error of one sort or another.)

Testing SMTP AUTH via telnet

PLAIN method

First, we need to generate a base64-encoded string for the PLAIN method, in the form of [null]username[null]password :

# perl -MMIME::Base64 -e 'print encode_base64("\0user\@domain.com\0password");' 
AHVzZXJAZG9tYWluLmNvbQBwYXNzd29yZA==

IMPORTANT NOTE: notice that we escaped the '@' character with a backslash! If you don't, Perl will try to evaluate it as an array, and while you won't get an error, you won't get a usable hash, either.

OK, let's telnet in:

ph34r# telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.privatedns.com.
Escape character is '^]'.
220 mail.server.local ESMTP

Great, we got a banner. OK, now let's tell it we want to use extended SMTP with the "ehlo" command:

ehlo test
250-mail.server.local
250-AUTH LOGIN PLAIN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-PIPELINING
250 8BITMIME

Alright. Good. Notice that we support two AUTH methods: LOGIN, and PLAIN. We'll issue an AUTH PLAIN command, using the string we generated for it above:

AUTH PLAIN AHVzZXJAZG9tYWluLmNvbQBwYXNzd29yZA==
235 ok, go ahead (#2.0.0)

Excellent! Our authentication proceeded just fine. If we would like, we can now send a message just as we would have in the regular SMTP session listed above; or we can simply quit now if all we wanted to test was the authentication itself.

LOGIN method

If we want to try the LOGIN method, we'll need to generate separate base64-encoded strings for the username and the password:

ph34r# perl -MMIME::Base64 -e 'print encode_base64("user\@domain.com");' 
dXNlckBkb21haW4uY29t
ph34r# perl -MMIME::Base64 -e 'print encode_base64("password");' 
cGFzc3dvcmQ=

Now we telnet back in, ehlo as before, and this time authenticate using the LOGIN method:

AUTH LOGIN
334 VXNlcm5hbWU6
dXNlckBkb21haW4uY29t
334 UGFzc3dvcmQ6
cGFzc3dvcmQ=
235 2.0.0 Authentication successful

Again, excellent. (If you were curious, the 334 and 235 SMTP messages are also Base64 encoded, and decode to "Username:" and "Password:".) You can either send a test message with the same commands as in the plain SMTP example at the top, or you can simply quit if you just wanted to test the authentication.

See Also

Daemons, testing via Telnet IMAP, testing via Telnet POP3, testing via Telnet