Sometimes, you need to test SMTP auth (for sending e-mails) is working properly and you don't want (or can't) test with an ordinary email client.

One can test using a telnet session. But first, you must encode username and password using this command snipplet:

echo -en "testlogin" | openssl enc -base64
dGVzdGxvZ2lu
echo -en "testpass" | openssl enc -base64
dGVzdHBhc3M=

Then:

telnet <your_server_address> 25 (or 587)

Now you do the same as an e-mail client:

HELO mybox.mydomain.tld
250 host.domain.tld
AUTH LOGIN
334 VXNlcm5hbWU6
dGVzdGxvZ2lu
334 UGFzc3dvcmQ6
dGVzdHBhc3M=
235 2.7.0 Authentication successful
quit
221 2.0.0 Bye

If something with "Authentication successful" appears, login was able to authenticate against the mail server for sending e-mail.

REMARK: There are some other sites with examples in perl that don't work with full e-mail address usernames (user@domain.tld) because of lack of escaping the "@" sign that designates a perl array.