Allow admins to limit messages between users

Historically, other forum software contain the option to limit how many messages can be sent between users. One reason this is done is because some users have different paid classes (thats another issue). But it could also be a performance issue. If you have 5000 members on your site, and then many of them are sending messages back and forth (say 400 a year), the site would grow to 2 million messages a year.

Many sites have a limit for new or unpaid users to only 50 messages total.