Mine did the same thing a while back. I had to go to User CP > Edit Options> and chane the 'Default Thread Subscription' to 'no email notification', save, then back to 'instant email notification' and save again. Fixed it for me.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry