Can't send text message to one contact


I am suddenly unable to send text messages to one contact.  He can send me messages, but when I reply or send him a message, I get a message saying "Text message not sent."  I can call him.  I can even send him MMS messages (if I attach a picture to a message, it goes through). 

I am able to send text messages to anyone else.  He checked his blocked numbers, and mine is not among them.  Any ideas on what is going on?

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