two things might have happend..
one is a spammer somewhere has sent a load of emails out with your email address faked as the originating address, so any that are sent to non-existent addresses the non delivery replies are coming back to you. It's the same as when you receive normal spam you can pretty much guarantee that the reply address is fake Unfortunately theres not a whole lot you can do about this reason.. they will probably stop after a couple of days though.
The second and far more serious reason this may happen is if your computer has become infected with a virus or spyware that is causing it to actually send out thousands of emails
just in case it's the latter reason you should be running a full system scan with your anti virus software (please say you do have some!), and if you haven't already got some, get some anti spyware software and run a full scan with that. get adaware free from the link below if you haven't got anti spyware software
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