Do you share 100+ files? I'm asking as if you are not sharing that much, it could mean that you are ending up in ignore lists as they will think you are a leecher.
Sometimes, people just don't want to recieve any messages, so they will make sure that no one can message them by making the appropiate settings.
If you have been ignored, then the message that you are being ignored by the user will come up.
You can't do anything in your side. The messages won't get to them as they don't want any.
You can change you name slightly and then you can message them eg:
If your name(in winmx) in abc, change it to abcd and you'll get out of that user's ignore list
Cheers