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 mailserver.com 25

Greet the mail server:

EHLO mailserver.com

Tell the server you want to authenticate with it:

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:

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:

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:

user@localhost [~]# telnet exampledomain.com 25
Trying 1.1.1.1...
Connected to exampledomain.com (1.1.1.1).
Escape character is '^]'.
220-server1.exampledomain.com 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 exampledomain.com
250-server1.exampledomain.com Hello  [1.1.1.2]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
AUTH LOGIN
334 VXNlcm5hbWU6
dXNlcm5hbWUuY29t
334 UGFzc3dvcmQ6
bXlwYXNzd29yZA==

235 Authentication succeeded

[출처] https://www.ndchost.com/wiki/mail/test-smtp-auth-telnet