I have a white iphone 4s 16 gig (verizon and non jailbroken) and was having same issue of not being able to send MMS, tried all solutions (data on, reset, *228, blah blah) with no sucess... I then went through recently dowloaded apps, and deleted Onavo Extend (data compressing app-similar to opera mini) after uninstalling, my iphone started sending MMS...also got a s**tload of pictures from multiple contacts from like the two weeks it wasn't working all at once. I"m not sure what the correlation is, but maybe data compressing or data altering apps have some effect on MMS. All i know is it solved the issue for me, and havnt had a problem since, just trying to help out others that have this extremely frustrating problem! Anyone please reply if this works for you as i'm still curious if this was just my device or if it will work for everyone. Goodluck!!