I got a message that my account appeared to have been compromised. So when I logged in, just for the heck of it, I checked my "sent email" folder and saw that, starting Sept. 2011, there had been several batches of hundreds of emails sent out to people that I didn't know, containing what appears to be spammy links. (Didn't click on the links.) It happened most recently on March 5, as far as I can tell. It seems to only happen every few months.
I don't know why Yahoo notified me today that my account appeared to have been compromised - not sure what tipped them off.
I have NO idea how this happened! I am the only one who uses my computer. I do have a wireless router - not sure if that increases my security risk somehow.
So, I went and checked all of my other email accounts and thank God they were fine.
I also changed my Yahoo email password. Hopefully that works. I'm tempted to just cancel the account completely.
And in the meantime, I urge everyone to check all of their email accounts "sent mail" folders, to make sure your email account hasn't been blasting out spam without your knowledge. I'm very lucky my account wasn't used to spam friends or family or clients, just weird random email addresses.