Originally Posted by 3M TA3

I run it on all the vBulletin installs that I use and I don't see a major use of resources from it.

You can disable members from using PM in vBulletin until they reach a certian post count. You can also limit the number of PMs that someone can send at one time. It's not worth it for a spammer to send one PM to each person vs bulk.

You can allow members to turn off the ability to receive PMs and/or email notifications of the PM or restrict them from being able to change that setting.