This might help, although take into consideration this was techies, so it's second-hand info and from the carrier, NOT RIM.
I was told that EVERYTHING goes through RIM's network, and if it's from a BB, it likely will go through, but if it's from/to another phone, I guess RIM edits them or only allows certain types etc., I didn't even get the details.
Bottom Line: If the person doesn't have a BB, MMS might not work - use e-mail, it works better