IPhones IMessage isn't working?

If I'm texting someone who doesn't have an iphone my texts will send perfectly fine but if I text someone with an IPhone and use IMessage it will say my message delivered and then a couple seconds later say "Not Delivered" under the message and I have to send it as a text but that person won't even get that. I can still get texts from people using IMessage but not send them. I've tried turning my phone on and off and disconnecting to Wi-Fi and then connecting to it again, neither worked.

Back up your phone and restore it. But first go to settings-general-reset network settings.

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