ok....this sounds like something the Klez virus does. It sends emails to people in your address book, but changes the return email address to make it look like it was from someone else in your address book. SS, that postmaster email from yahoo that you got is very similar to things that this virus does also.

I advise everyone with problems or funny emails to check this link for info regarding this particular virus:
http://[email protected]

you can also download a tool that will remove this particular virus. better to be safe than sorry i say

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