can you run something like iperf3 or openspeedtest between the server and client to prove its a network throughput issue?
do you have a network switch you can add to avoid switching through your router (if it is indeed bad?)
Have you ensured you arent unknowingly using wifi at either end?
I have .solutions and .info domain emails that still gets denied by some services, especially anything government or public utility, pain in the arse.
You’d think that at least .info would be pretty well accepted by now.