My iPhone won't send iMessages

For some reason my iMessage won't work with some people. It will just send as a regular text message even though the other person is using iMessage and I have iMessage enabled on my phone. Anyone know how to fix this?

Android

Try enabling both Imessage and SMS Messaging. That way if they have something other then iphone they can receive with no problem and you can as well. If don't help Reset Network Settings

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