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Are you using the stock messaging app or some 3rd party app?
If you are using the stock app I suggest that you try to repair the phone software using PC Companion..
Before repairing your device you may want to backup your information first. Check out this topic for more information on how to.
How to backup?
If the issue should still remain I think that this needs to be examined and fixed at a repair center. For more information about how to submit your phone for repair and where, contact your local support team.
Before a software repair,try settings-apps-all-messages,force stop and clear data.
So even a software repair didn't help?
Software repair is different.Could you give it a a try?
http://www.sonymobile.com/gb/tools/update-service/
It is easy.Download the above application,install it and follow the steps provided.