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Ok how did it get all the address from my address book ?
Are you sure that all those people got messages? Or just those who are mutual friends of someone in your list.
I'm betting that the only addresses that were in your address book that got these messages are the ones that were also in the address book of the friend who has the worm. The mutual friends who told you they got the messages think they came from you because your name is in the "From:" section. As far as I know, there is no known worm, in the wild, that will operate on the Mac OS. However, there are known trojans. But, all bets are off, if you download and install one of those.
Of course, if you are also running Windows (dual boot) that OS can be infected, just like any other Windows machine. The good part, is that even if that partition is infected, it still won't bother your Mac area.
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Thunderbird. It's AT&T Yahoo mail.
Oops!
I completely missed that. So, it's not the address book that is on your Mac. It's the one on the Yahoo server. Never kept an address book off my local drive...no control and all that...same reason I don't store my important files on-line! At any rate, there's definitely no worm 'burrowing' on your Mac.
BTW, by not downloading your messages, they are still on the server at Yahoo. That can be handy if you travel with one computer and normally use another at home. OTOH, you may actually be downloading your messages and have set up Thunderbird to delete them in a certain amount of time.
IMAP stands for Internet Multiple Access Protocol. No, wait...it's Interesting Mail Accumulation Process. NOT!
I really don't even care enough to look it up!
While I have one IMAP account (mac.com) I use their POP server and when I read a message on my iMac, but the IMAP server when I access it from the iPhone. The advantage is you don't have to worry about your mail box filling up and not getting messages because of that. Of course, that's not much of an advantage now with most ISPs offering ever more space.