Suggestion:
Make so that you cannot click on this:
If the user has a full pm box.
Normally it would let you click on it, then you would type your message to that user. When you're done typing your message you realize that you cannot send it because their message box is full.
So it would be better that it warned you beforehand that their box is full so that you can come back to their profile and message them when they have cleaned their box.
If it's unclear what I mean, please let me know and I will do my best to explain what I mean
.