In my case, it doesn't appear to be connected to mail.
The popup always occures just when I close out another web site by using the little X in the upper right. I even thought I was inadvertantly clicking on something for a while, but that is not the case.
At least I now know that it isn't just me.
I think I will inform AOL that I probably wouldn't be having this problem if I were using another ISP. Maybe that'll jog them enough to come up with a cure.