Do you have a yahoo mail account?
Or another web-based mail account?
I know this happened to a group of friends of mine who had yahoo mail accounts (including AT&T accounts which use yahoo as a mail provider). Everyone in their address books was getting these emails. One of them had a “sent mail” folder that keeps a copy of every email that was sent from their account and, sure enough, the mail was in there.
Changing their yahoo mail passwords did the trick. Of course, I told them all to run malwarebytes and whatever, but there was nothing resident on their machines. What appeared to have happened was someone cracked the yahoo mail passwords. This is just speculation, yahoo is not admitting anything of the sort.