Here is how to test SMTP authentication with telnet using terminal and openssl in Darwin or Linux:

1. Get your user and password in base64 encoded strings (you need them in base64 because the server wants them to be like that)
   # echo 'myuser' | openssl base64
   # echo 'mypassword' | openssl base64
2. Open a telnet conneciton susing port 25:
   # telnet myserver.domain.com 25
3. Greet the server
   # EHLO myserver.domain.com
4. Tell the server you want to authenticate
   # AUTH LOGIN
5. The server now asks you in a base64 encoded string to enter user. Enter the encoded string you received for 'myuser' in point 1
6. The server now asks you for the password, again in a base64 encoded string. Enter the encoded password received from point 1
7. You should see a message now with authentication succeeded or authentication failed