When I first upgraded to 18.2 I added --ssloption "ssl_op_allow_tls_v1_1,ssl_op_allow_tlsv1" to my POA configs to support Macs. It was working and didn't think anything more of it. Tuesday I upgraded my last remaining domain and PO to 2018.2 and much of my environment stopped working. The POAs would fail to connect to the MTAs. Finally I added the switch to my MTAs and the POAs began to work again. I had to update every POA, MTA and GWIA to allow tls1 before mail was working again.
This whole thing would have been avoided if either there was a new Mac client or if the new Web client that's still in development was featured completely enough to replace the 10 year Mac client.