2018.04.05 08:09

How to Test SMTP AUTH using Telnet

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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:

perl -MMIME::Base64 -e 'print encode_base64("username");' 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:

telnet 25

Greet the mail server:


Tell the server you want to authenticate with it:


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:


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:


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:

user@localhost [~]# telnet 25 Trying Connected to ( Escape character is '^]'. ESMTP Exim 4.66 #1 Wed, 09 May 2007 23:55:12 +0200 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail. EHLO Hello [] 250-SIZE 52428800 250-PIPELINING 250-AUTH PLAIN LOGIN 250-STARTTLS 250 HELP AUTH LOGIN 334 VXNlcm5hbWU6 dXNlcm5hbWUuY29t 334 UGFzc3dvcmQ6 bXlwYXNzd29yZA== 235 Authentication succeeded



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