I recently sold one of my iPhone 4's to a guy on Craigslist. He says he can't send MMS, and I think it's because he doesn't have a data plan on his phone. I'm not really sure though, but could this be the issue? I've also noticed my phone has some issues with sending MMS messages. Occasionally the feature won't work, but I can receive them 100% of the time with no issues. Both phones are of course jailbroken.