Troubleshooting an Exchange Hybrid mail flow issue where inbound mail is failing to route through the F5 appliance. The overall network setup is Exchange Online <-> Palo Alto NGFW <-> F5 LTM <-> Exchange Pool. By default, Exchange Online will attempt to secure the connection over TCP 25 using TLS 1.2, and it seems this is where the issue is taking place. The F5 virtual server configuration is very straightforward, and I'm attempting to configure it to support SSL Passthrough (not Bridging or Offload). The VS is listening on TCP 25 and is performing a single forward to a backend pool, which I've limited to a known good working Exchange Server. No Client/Server SSL profiles have been configured (i.e., Passthrough) on the virtual server.
A traffic capture on the virtual server does not show any STARTTLS negotiation taking place, which supports the TLS error we're receiving on the Exchange Online side. As a test, I've moved the flow of traffic around the F5 to allow direct communication between Exchange Online <-> Palo Alto NGFW <-> Exchange Server, and this is operational, and I can see the TLS negotiation taking place.
I've referenced the SMTP deployment guide particularly for the Passthrough configuration option, and everything (other than the port 587 not 25) is correct. Both Exchange Online and the Exchange Server will require TLS, but configuring the F5 in bridging mode will not work as we do not have the private key of Exchange Online.