Tigerdango hit the nail on the head. The problem here most likely lies with the iPhone itself. I used to have an iPhone 3g (8gig) (unlocked and jailbroken) on T-Mobile's network. There are so many angles to cover here: Which version of Apple's software they're running? Have they unlocked or jailbroken? If so have they installed 3rd party applications to install SMS and MMS settings for them or have they done the homework to manually program the phone? If they did, was it correct? etc. etc.
iPhones have had poor history with MMS until iPhone 4. When I got mine it didn't even have the option to MMS at all, I had to manually program it to get it, and even then it was hit and miss. Most of the time I could send but not receive, and when I sent them out it took FOREVER. (This is because if you're using an iPhone on T-Mobile you don't have 3G, just EDGE, which is hella slow.)
There are parameters you have to set in the iPhone that "govern" what type of data and messages it can send and receive. Unless these are 100% correct, the phone will fail every time, and this includes the correct image size parameters, apn's, etc. These also differ depending on which software version the phone is running... I stayed with 3.1.3 because 4.0.1 made my phone too slow.
IMAGE SIZE IS HUGE! Don't expect to be able to send your friend with an iPhone your amazing 8 megapixel photos, the iPhone just won't know what to do with it. Chances are their getting an image size error message on every attempt. You'll have to at least dumb it down 3MP, remember, the smaller the easier the transfer.
I will note here that I'm not sure how this effects iPhone 4 because I've never owned one, so I'll leave iP4 alone.
I would venture to guess that it's the iPhone, not your INC. Hope this helps.
