I think you should be aloud to message anybody, and if they annoy you or spam you with messages, they should also be some sort of ignore list you can move them to, or even block them.
The only real necessary addition that I believe is needed for the messaging system is just an aesthetic one: pagination. It would decrease lag significantly making it easier for users on mobile devices to access the page without difficulty.
It's my job to help the next generation, and set a good example for them. And for that I'll gladly lay down my life.
