This is a VERY important thing to be able to do and has saved my bacon on several occasions!
So FOOEY to all those who voted down this question! PLEASE NOTE: I'm referring to the technical question, not the motive. Being able to do this is important for testing ones system, for example. Having the system up and therefore able to use an automated web-based open relay testing site is not always possible.
You also said, "I actually want to know how can i do this myself.."
Well, OK, the first thing to point out is that there's an RFC on this one - the official specification, and you can find it and have all the data you need.
Here are some basics:
telnet **domain.address** 25
(25 is the standard port, but it could be set to something else)
MAIL FROM: **your.email@address**
RCPT TO: **firstname.lastname@example.org**
now you can type your message.
6) You may now, optionally, enter a subject line like this:
**Subject:** whatever your subject line is
(there is no response to this)
7) When done, end with:
(In other words, new line, period, new line.)
6) You can now enter a new mail message if it's destined for this same
server, or you can exit by entering: