I do an import from threads 6.5.5 so my user have lot of old PM's in the inbox. I set the PM limit to 200 and several user are over limit.
If a user send out a PM to such a overlimt user the PM was send without any notice and you see (as sender) while reding your PT the this PM is unread by none (instead of the username). This PM seems to be discard without notice.

Suggestion:
If a user anter his PM box you should count how much he have. If he is oferlimit popup a warning box and notice him to delete some old ones.
The sender of of PM to an overlimit user should be notified that he cant do it because the reciver box is full.

I concour... Warnings should be sent to both the sender (with "The target user [user]'s inbox is full") and a popup to the user whos pm box is full... Possibly even an icon on the navigation bar like the "new pm" icon

Revisiting this. When you click the 'Add Recipient' button it does check to see if they are over limit. It appears if you're just sending to one and don't it doesn't do the check. Need to fix that for 7.0.2.