Why is my iphone 4s sending texts to everyone except one person in specific?

I have an iPhone 4S, I've been texting a few people today but there's this one person in specific that my phone doesn't let my message go through to them? It just randomly happened in the middle of one of our conversations when I tried to reply but all it says is "message send failure." I didn't want to, but I deleted our whole message history and still nothing.

Could be their service

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