I am on AT&T and my iPhone 4 is stock with 4.1, no JB or unlocking, and I haven't been able to send any MMS for the past 2 weeks. At first AT&T thought it was the network in my area so they gave me a 3G MicroCell at no cost to fix the problem, but even when connected to the MicroCell I can't send any MMS. I can receive all of the test MMS messages that the AT&T tech support people send my, but even immediately after receiving an MMS I still can't send one to anybody. I get the same thing where the progress bar stops at the end and just sits there until the "Unable to Send" symbol pops up.