Using telnet to send e-mail is about as geeky as I get, but it is useful to know how in order to test for an open relay on your mail server. An open relay would allow anyone to send mail from your mail server and is an easy way to get yourself blacklisted. So put the pen back in the pocket protector and lets get to it!
telnet [mailserver] 25 and press enter. You should see a message like 220 mail.domain.com ESMTP blah blah blah. That means you are in! Now its time to be polite 🙂
Helo domain.com and your host will acknowledge your presence with 250 helo 192.168.1.1 or whatever your IP is. You may also see some other information as well as part of the acknowledgement.
mail from:email@example.com If all is good you will see a 250 2.1.0 Sender OK reply
rcpt to:firstname.lastname@example.org Again you will get a similar reply saying the recipient is OK.
data After you type this and press enter you can type your message.
To end the message and send it press Enter . Enter and you will be notified your message has been queued for delivery. One thing to note, spelling mistakes are not tolerated, even going back to correct it will end in an error message, however, using uppercase or lowercase is fine.
Now if your mail server has been properly configured so that it is NOT and opne relay this should fail if you are not authenticated against the domain.