Is the sender's message included in your response? Anything in that which might cause an issue?
When I was arranging for a roofer to do some work for me, I was unable to send their office anything by email other than my original reply accepting their quote (original request for a quote was in person as I passed by their office regularly!). No idea why, all subsequent emails sent by both laptop and phone bounced back, so text/visit became our comms route.