Demirmkn,
There is no support by email for this, you must call them and report the problem by phone. You can call them for free from a @gmail.com account, using Google Talk integrated in browser. The phone number to call is +1-800-694-7466.
As Nav33d here already outlined the list of the things we have tried already, so you know what to tell to support, as they will ask you to do these things:
1) Calling Sprint (you probably paid the 'unlocked' price and as a result won't have an account with them so they know nothing about you/your phone).
2) Calling your carrier (if they're outside the U.S). The problem is with the phone having a lock on outgoing SMS.
3) Resetting Network settings.
4) Using the *#5005*7672# trick to reset the message centre number.
5) Restoring from iTunes/Backup.
6) Restoring as a new phone.
7) Removing sim, putting sim back in.
8) Resetting phone.
9) Tinkering with iMessage/MMS settings.