How to Test SMTP AUTH using Telnet

Below are instructions on how to test SMTP AUTH against a mail server using Telnet and entering the commands by hand.

The first thing you need to do is get a base64 encoding of your username and password. There are a couple ways to do this, the example below uses Perl:

1. perl -MMIME::Base64 -e 'print encode_base64("username");'
2. perl -MMIME::Base64 -e 'print encode_base64("password");'

What will be returned from each command is a base64 encoding of the username and password; save these as you will need them later. Now connect to the mail server using Telnet:

1. telnet mailserver.com 25
2. Greet the mail server:

1. EHLO mailserver.com

Tell the server you want to authenticate with it:

1. AUTH LOGIN

The server should have returned 334 VXNlcm5hbWU6; this is a base64 encoded string asking you for your username, paste the base64 encoded username you created earlier, example:

1. dXNlcm5hbWUuY29t

Now the server should have returned 334 UGFzc3dvcmQ6;. Again this is a base64 encoded string now asking for your password, paste the base64 encoded password you created, example:

1. bXlwYXNzd29yZA==

Now you should have received a message telling you that you successfully authenticated. If it failed your user/pass may have been wrong or your mailserver is broken.

Below is a log of a real successful SMTP AUTH connection over Telnet:

1. user@localhost [~]# telnet exampledomain.com 25
2. Trying 1.1.1.1...
3. Connected to exampledomain.com (1.1.1.1).
4. Escape character is '^]'.
5. 220-server1.exampledomain.com ESMTP Exim 4.66 #1 Wed, 09 May 2007 23:55:12 +0200
6. 220-We do not authorize the use of this system to transport unsolicited,
7. 220 and/or bulk e-mail.
8. EHLO exampledomain.com
9. 250-server1.exampledomain.com Hello  [1.1.1.2]
10. 250-SIZE 52428800
11. 250-PIPELINING
12. 250-AUTH PLAIN LOGIN
13. 250-STARTTLS
14. 250 HELP
15. AUTH LOGIN
16. 334 VXNlcm5hbWU6
17. dXNlcm5hbWUuY29t
18. 334 UGFzc3dvcmQ6
19. bXlwYXNzd29yZA==
20. 
    
21. 235 Authentication succeeded