Yahoo! Mail Contact List hacked...

I had what I think is the same thing that happened to you, except it was my Yahoo! Mail Contact List that was hacked. If I understand correctly, that contact list is kept on Yahoo's servers, not on my computer. Normally I use Thunderbird and the address book that is on my home PC; the Yahoo! Mail list was a very abbreviated version of my full address book. I did not know of the hacking until one of my correspondents sent me the message he had received, which included the entire group of recipients of the spam (which was much as you described).

At first I thought my Thunderbird address book had been hacked, but not so. So this appears to me to be a problem of Yahoo's servers being hacked.

Incidentally, more than half the contact list contained inactive addresses, which generated bounce messages back to my Yahoo account, but they were directed to the Spam folder on my Yahoo web mail, which I only check every two weeks or so.

I immediately deleted all the contents of my Yahoo! Mail Contact List, and will NOT be using it again. Not that MY list would ever be broken into again. I informed Yahoo of the problem, but they seemed oblivious to the fact that it was their servers that had been hacked, not my computer.

Or do I have the wrong idea of what happened?