Messages to one person showing not delivered, but they receive them

[dgdubra]

So hubby and I have the same phone , Iphone 4s.  Starting the this past weekend, when we send text messages to each other ( and only each other, not even others with iphone 4s or otherwise ) it comes back that the message is " not delivered "  however we get the txts. 

This happens on EVERY txt we send back and forth, and to both of us, but on each phone, the only txtx affected are the ones to each other.

Any ideas, why?  Why only one person? How to fix?

Thank you,