Must be something to do with vbulletin then or php/windows update?, I don't receive anything at the other end, checked everything I can think of, I can try several different mail accounts if needed, let me know if you I can help with anything dude.
Googling only yielded this..
http://www.vbulletin.com/forum/forum...ing-emails-out
Cant be blacklisting as other mails are working you say.