[AngelBlue]

Two comments.
First, I would suggest that the default error for users affected by this mod be changed to :

Originally Posted by new_phrase_suggestion

Sorry, but you may only send private messages to forum administrators and moderators.
This limit is due to your post count ({2}) being very low.
Once you have reached a post count of {1}, the limit will be removed and you will be able to send messages freely.

The second thing I would suggest is that when this mod is installed, someone who is limited to sending pms only to staff should get an error when they click on the "send PM to..." link in a non-staff member's profile. This would stop them wasting their time typing a message only to be frustrated because the system won't deliver it.
Even better would be if the link to send a PM diddn't even appear to them, but was replaced by a "Contact this user via privmsg (requires X posts)".
Best of all would be if:
* there was an admincp option to replace the "send PM to..." link in profiles with custom text for people affeted by this mod
* there was an admincp option to give the same treatment to email addresses and other contact details in profiles