I was having this problem for a week or so, and I finally figured it out. I thought I would post it just in case it could help someone else.

I was only receiving emails from vbulletin on the half hour (i would get a group of emails on the :00 of every hour and on the :30 of every hour)

I finally figured out, when I enabled the -f option in server optimization section of the admin cp, the problem was resolved.

Once I enabled this feature and it solved my problems.

Hope this helps others