Is it only SMS messages from Myxer that are not being received? Can you receive messages from other people on their phones?
Generally, if you can send, but not receive SMS messages, that is indicative of a message box filled to capacity. Houseclean your message box. In Messaging, tap your Menu icon and select Delete Threads.
If you are able to receive from all but Myxer, I would be looking for a setting on Myxer. The phone doesn't care who the message comes from. It only cares that it is a message in the correct protocol format, and that is up to the sending source.
Samsung r880 Acclaim/i500 Mesmerize
If I've helped you, please give me thanks. I'll do the same for you!