Hello pmc,
Im not sure this would be usefull but its just an idea.
I noticed that there are a lot ways to report abuse but if we are talking about private messages there is not a system for that and if something happens with that we have to send a private message to a moderator. I actually never needed it but maybe other people can use it if they need it.

My suggestion:

My suggestion actually works the same as the report system for whispers.
Just a report button for every message someone gets, if people press that button they get a popup:

In that popup people could choose some options:
-The person who made the message is harrassing other members with the message system.

-The member is talking about inappropriate subjects or using profanity.

-The member who made the message is advertising things without permission.


And a field the member can add things they want to say about it or add other things to make the report more complete.


After that the member can press "send report" to complete the report, a copy of the message will be send to a moderator. They can look at it and take actions.