I think it's on the recipient's end, but I could be wrong. I am on Verizon and I have a friend on ATT that never gets my MMS. What carrier are you? Also, did the person have an iPhone?
You never said which carrier you have? Do you have Verizon? You don't think that the problem is on the recipient's end? ie..you friend with Sprint. Is this a regular occurrence? Same person? Try sending an MMS to some folks on yet another network and see what happens. AT&T for instance.
If both those on Sprint have the same problem as you described, I would think it's not your phone's problem. Call Verizon and explain the issue and see what they say. They have been very helpful for me, and if they can't help, they can put you through to Apple Support. But I think it's with the Sprint phones.