I have some suggestions regarding personal messages.

My suggestions are based on problems I face managing a 14,000 users website. Some stuff that I have to fight against are: users trying to sell products by PM and companies trying to do marketing research with my users without authorization.

So.. What I suggest for PM is doing restrictions, just like the group permissions. So, depending on the user group, a user will be able (or not) to send/get PM. Even better If we could difine the PM mailbox size depending on the group. 

This way I could be able do do something like:

New user groups - Cannot receive or send PM.
Begginers group - Mailbox for 10 PM, but can not send.
Intermediate group - Can send mesages and have a 20 messages mailbox.
And so on...

Also have the personal permission: block PMs access for a specific user.

This would solve my problems... especially the "PM spam".

Another suggestion would be to allow moderadors and/or admins, to have user groups for announcements. So, If we change our forum rules, for example, we could be able to notify all users by PM.  Or announce something for a specific group of users (no user groups, but group of pre-selected users.)

This would be much better than trying to email all users, because would not be considered SPAM, would not spend external network resources and would just add some database row, not even a big grow in database size (even because users can delete it latter). The only thing to take care is to disallow any email report for this messages (of course!).

Another idea that would be really useful for me would be to have PM notifications for new topics created in a specific forum. In other words, a user could receive a PM when a new topic is created in a specific forum.

The scenario is:

I have a Official Announcement forum, where the moderators/admin rarely post topics. This mean that this is not a popular forum, although it is important. So, what happens is that when a topic is posted, the users doesn´t see it, because they are not used to check this forum.

Another situation is that we have a suggestions/question forum that should be checked by the moderators eventually. But since there are not a new post for weeks, the moderators forget to check it. So the users thinks that they doesn´t care for suggestions.

And a third situation is that we want to allow some companies to give support for their products in a specific forum. So, instead of them checking for new topics in a regular basis on a specific forum, they would PMs.

Anyway... 14,000 rows is not a big problem. This would not increase the database that much. Even If we have these messagens in a regular base, the non active users will soon or later have their PM mailboxes full and not get more messages and the active users will probably delete the announcements after reading it... even that... a bad scenario would be a 60.000 rows increase each month ? how much this represent in megas for database grow ?

Even in a much worse scenario... If the database grow and I have to use this function I´ll have to pay for a bigger hosting plan/database... this is ok for me as long as it is useful. I´ll be paying for a better service for my users.