HTC One M8 Not sending messages

[Ph1r3]

Recently, my HTC one m8 has stopped sending messages to just one person. What I don't understand is this particular person can message me. I tried everything that I found here and contacted my cell provider. I did transfer the phone from Verizon to t mobile about 4 months agof, and this issue just started 2 days ago. Help!

 

[the tall guy]

What device does the recipient have? are they on Imessenger or were you?