This is ridiculous - I'm experiencing the same problem again, as have hundreds if not thousands of other Nokia S60 users across a large number of different phones. Having the "accepted solution" of all of these be a hard reset of the phone is just plain nuts. This is a SERIOUS issue that has plagued Nokia phones for years as far as I can tell, and there still is no real solution. I don't want to do a hard reset - I want my phone to just work like it is supposed to.
I too have looked at all the other settings - my message center number is correct, creating a new one with the same number did no good, etc. This is not a provider issue - it is a Nokia issue.
The first time it happened the phone crashed just after I had sent a text message - it then continued to resend that same message over 40 times to my fiancee's phone before I figured out that it was stuck in the outbox resending every three minutes. I found I could delete the message, and keep it from sending over and over - but now my phone would not send any new messages, although it could still recieve them. I pulled the battery and the sim card, and eventually my ability to send text messages was restored - no longer would they sit in the Outbox with "resending" next to them. Then a couple weeks later it happened again - but this time with no crash of the phone first. Once again I was able to get my outgoing texts back by jumping through hoops. Now, a few days later, it has again popped up, and so far, I can't restore my ability to send messages.
So I could back up my phone and do a hard reset - but what is my guarantee that this is going to solve the problem forever? Oh that's right, it doesn't. Maybe I will get my ability to send messages back for a week, a month, etc. Or maybe a day. I'm just fed up with this issue. I could put up with a bunch of other problems that the N85 has, but this just takes the cake - because it isn't just an issue with the N85. It is endemic to all Nokia S60 phones as far as I can tell, from reading so many different accounts of the same issue. I can't wait for an Android phone to come to AT&T so I can dump this thing. I loved my old 6230 S40 phone - it just worked. But S60 just seems to have so many issues!