well from what i found out with my iphone 5, if you have a failed message, it in a way sits in the outbox clogging up all following messages... here is my story with what i learned, at first i was so irritated, but now it kindof makes sense.
bought new iphone 5, hour after activated, i texted my best friend first with it. for some reason text message failed, i didnt know this though, just thought he didnt reply. about 4 hours later i texted other people and they got my messages just fine and replied just fine.... all up until today that one friend could never get my messages, we tested and tested and tested sitting next to eachother and everything. it boiled down to he could text me, but when i replied he would get nothing. then we both would be confused, him because i didnt answer his question, me because he wasnt replying to my answer lol.
what happened was that first message that failed, prevented all of my other messages from going through, they would show sent and would look like they went out, but wouldnt actually go throught, this did not stop his text meassages from getting to me, just one way... what i actually did was went to messages hit edit, and deleted the entire thread i had with him. after doing this it allowed my messages to go through just fine.
i realized this last night when i was in mid conversation with someone, sent them 3 pictures and noticed only one went through, it was pretty much at that point where they stopped getting my messages and after a few more messages i could tell, they actually were not getting my messages, i called them, sure enough it happened again.... not sure why it does this, but chalk it up to another iphone failure lol. maybe with an update messaging will be fixed.