i had this too. agree with all that was said but have a bit of info to add.
I am a yahoo paying customer. i recieved the same returned e-mails. i sent them to the yahoo abuse department. they said exactly what was said above...that spammers are searching for e-mails to use as reply to addresses to lend validity to them....and the sad thing is there is nothing they can do about it because the e-mail never touches their server until it hits it's destination.....my inbox. unless of course it is not returned tome...then yahoo never have any contact with it whatsoever.