I dont recall the lat time I got that kind of message.
I would start with making sure I have correct number of the person I am sending a text to. To simplify, perhaps have them text you. Then you can try to reply to them. That way, if it came in correctly, the reply will go out correctly.
If that fails, you may have a problem with your APN settings. If that is the case, we may be able to help you or you may have to call your carrier for the proper APN settings.