I just found out that my email address has been used to send spam messages...how do I stop this?

Change your password from a different machine, but chances are they're just spoofing your address.  It happened to me with my old hotmail account to the tune of hundreds of bounced-back emails per day.  I had to abandon it, and gave up on them because they wouldn't help me.

Can you check to see where the messages are being sent from?

I presume they are not coming from a compromised mail server or a malware infected machine, but rather some spammer is just using your address as the "from" line.

Happens to me a couple times a year.  They send their spew and just randomly select one of the addresses to be the "from" address from that run.  You start to notice the problem when your mailbox starts to fill up with non delivery reports.

Not much you can do if this is the case, just wait it out and the scumbags will start using someone else's address.  The good news is most people will just let the spam drop in their spam folders and not bother with your address.

Now if you suspect your mail server has been compromised or you may have an infected desktop/laptop, more advice is warranted.

I changed my password for the email account. Seems to have solved the problem.

Had that happen to me on my hotmail account. Did it twice. Only found out when I started getting bounce-back from old/expired email addresses I hadn't cleared. Also, a bunch of people replying "WTF?"

I ended up dumping my entire contact list. That stopped it.

Yes.  I have gotten spam from myself.

So I wonder how many millions of people are targeting me right now ?

This in red is how you know that your E-mail was actually jacked instead of your address being spoofed. The password change will take care of it.