PM's going to wrong user
 

I have a client that has recently had another admin move her site from one server to another. After the server move she contacted me to handle the some other issues with the site and mentioned during our conversation that she was having an odd PM issue. What's happening is that some users (which as far as I can tell is random) are recieving PM's addressed to other users. What can I do to take care of this. I've searched everywhere and can't find even a hint of anyone else having this problem.