Why won't my iPhone send messages to non-iPhone users?

I've had my iPhone 4 for a while now, I go through Virgin Mobile. I've never had problems until recently, but it's really strange. I can text certain people with non-iPhones, but some I get an error. I can receive theirs always. Any resolutions?

Added (1). I have changed the contact from iPhone to just mobile… Still doesn't do the trick. I have turned off MMS. No such luck.

Its broken

Turn off MMS messaging in the settings for messaging. Also change your contacts that you are having problems with. I bet when you go to your contacts it lists their phone number in the iPhone category. Put the number you have for them in the mobile category when you edit your contacts.

Disable iMessage and check your service center settings which send messages out.