Sending e-mail with Telnet is really quite easy.

1. First of all go to Start -> Run, from here type telnet (alternatively go to command prompt and type telnet)

2. When telnet comes up type open mail.xxxx.com 25 (Where xxxx is the name of the server i.e. Yahoo and 25 the port)

3. After this type: helo smtp.mail.xxxx.com push enter you should get a message such as 250...............HELLO

Now from the email itself:

1. Type mail from:xxxx@xxxx.com (This is the address you want it to look from)

2. Now type rcpt to:xxxx@xxxx.com (This is who you are sending to)

3. For the message type data and just write your message here when finished writing press return then press . (The full stop button) and then return again you should now have a message saying message sent or qued.

SMTP Test Utility

Free, useful and detailed utility for testing e-mail/SMTP servers.
http://www.zoneedit.com/smtp.html