Are the individuals subscribed to the forums?
Go to your admincp -> email integration -> auto subscriptions:
And run the scripts there to ensure all the forum subs are updated
Lastly, double check your Vbulletin Options -> vbulletin options -> Email options and verify that you do not use the smtp authentication.
If after following those steps you still have issues, please PM me and admin login and I'll happily check out whats going on