is that about not having overlapping relays with the receiver (and different clients having different relay sets) or is it giftwrap in some clients and not others?
Discussion
coop only send message to user's nip-17 relays
Coop only does NIP-17 and NIP-17 is very clear: just giftwraps, NIP-44 and you must send the message specifically to the announced DM relays of the other person and receive messages at yours, so there is really no room for confusion.