Unauthorized usage of my email account.

I have seen this a few times. I think your email has been hacked. Change you password and it should stop.

this is one of the problem in using what is called a throw away account . after you change your password never access your Hotmail account through the computers at your local public library or any place like them . they do not have the security people need to be safe .

The issue regarding hacked hotmail accounts were presented all over the web.
Google query even brings up variety of howtos.

> because I didn't know who sent it

Well, Sarah Palin didn't either ;-)
The guy who was condemned now, said it took him few minutes finding out the answer to security question via websearch.
Prevention: gpg+brain1.0

You need to view the email with complete headers.
Lines in question are the 'Received: from...', where the last one is on top (usually from own mailserver, provider), so you need to read it backwards.
Look for implausible signs
e.g. does IP belong to domain, is the timestamp different to your mail provider and so on, compare headers of trusted and untrusted email to knowledge the differences.

If you find the abuse header line, you can forward the mail to e.g. abuse_at_domain, usually mainstream providers can receive it this way, or go to website of the domain and find the contact address for abuse.

http://en.wikipedia.org/wiki/Email
http://en.wikipedia.org/wiki/E-mail_authentication
http://wiki.answers.com/Q/How_do_you_stop_spoof_spam_email_from_your_own_email_address
http://clubhouse.microsoft.com/Public/Post/5b26dda1-16bb-4e08-9490-4d0e10fc7ad4

email is not a secure protocol -- the from field can be faked and sent from anywhere to anywhere -- it provides not guarantee that the person who is claiming to write it is actually who has written it. There is often no verification that the email actually originated from the computer who is claiming that it is sent is even originating from the correct side of the network.

If you want bullet-proof email, you have to use encryption or digital signatures to prove authorship, however most consider this to be too inconvenient to bother. (https://secure.wikimedia.org/wikipedia/en/wiki/Digital_signature). The reality is that most people simply delete emails that look suspicious, or sound like they were not written by the author (really bad grammar, obvious typos, difference in email presentation, unsolicited attachments).

P.S. Your email is unlikely to have been hacked, however changing your password may be prudent for a belt-and-braces approach.

You can send an email purporting to be from your account, to or someone else's account. There are valid reasons for doing this -- for example if you are using several mailing accounts, and want to make people reply to the correct account (eg forging your home email address when you send email to your friends from work -- replies should be sent to the home account (there is the reply-to field, but sometimes this is ignored)).