This technique is useful for sending mail under desperate circumstances, and it's pretty straightforward. From a command line, type something like the following. Note that type in boldface is what the server responds with.
telnet mail.whateverdomain.com 25
Connected to whateverdomain.com.
Escape character is '^]'.
220-whateverdomain.com Smail-18.104.22.168 (#2 1998-Jul-16) ready at Mon, 27 Nov 2000 13:
34:55 -0600 (EST)
250 yourdomain.com Hello yourmachine.yourdomain.com (yourdomain.com from address 22.214.171.124).
250 email@example.com ... Sender Okay
250 'firstname.lastname@example.org' Recipient Okay.
354 Enter mail, end with "." on a line by itself
From: Meredith Foo
Subject: Test Message
This is only a test.
250 Mail accepted
500 Command unrecognized
221 whateverdomain.com closing connection
Don't count on being able to spoof your from: address with this. Most SMTP servers capture your machine name and IP address, and that info is transmitted as part of the header of the message that you send. It's good enough for a laugh (email@example.com?) but that's about it.
Also note that the final SEND command may or may not be needed, depending on the server that you're using.
This only works for sending mail. For checking mail, see telnet to pop servers.