Before you can send an email to the server, you need to specify a couple of things, e.g. the from addy, the to addy etc. The way in which these are transmitted vary from protocol to protocol. However, if you use the most common method for sending mail (SMTP) then it needs to be in this format;

Helo // The equivalent of knocking at the door. Tells the server to get ready for data (not body data!)

Mail From // This specifies the address that you are going to be using to send the email

Data // This is the body text of your email, i.e. the Dear John part

It takes all of these parameters, and uses them to form an email. Sorry I haven't gone into too much detail, it's late over here, but if you want to check out some links for more info...

http://cie.bilkent.edu.tr/Topics/94.htm - A definition of SMTP
http://www.ietf.org/rfc/rfc0821.txt - The _long_ definition, read if you want to learn about the indepth technicalities (or you can't sleep...)
http://evolvedcode.net/content/doc_a...t/index_p5.asp - What I've just said, with some other details (and no, I didn't copy :P )

As for the legality of it, it really depends on your countries law. Technically your impersonating someone else if you manually enter the parameters (i.e. through Telnet) and the parameters you enter aren't your's. However, there's no real way to regulate this, as any attempts would probably overwhell the servers, not to mention be pretty impossible to actually think up of a test...