I had the same issues with my 8125. AT first, MMS messaging was fine, then I started gettng the error messages. Not from my phone, but the people who recieved them could not view them. At first I thought it was them, but then it couldn't have been everyone.

Alternativly, I had to send the pics to my laptop and email them that way. A solution that I found that worked was to do a hard reset on the phone. Only problem with that is that you loose everything on the phone, as it resets back to factory settings. So transfer all your important files first. When it does do a hard reset, it connects and sets up the 8525 to the 'lattest' AT&T wireless settings, and your MMS should work with no problems after that.

Hope that this helps and good luck