Why doesn't my Iphone let me send pictures to people without an Iphone?

I can send pictures through imessage, but not through a normal text message. Even when I try to send pictures as a normal text to people with iphones, I still can't send it. I have unlimited texting, and my settings say MMS is on. It doesn't give me any error messages, it just shows up with a red exclamation point and under the picture it says "not delivered." Anyone know why?

ITunes
contact customer care

Try to turn off your phone and turn it back on. It might fix the problem.