Error: Mailflow from Exchange Online to Exchange On-Prem doesn’t work.
Protocol: SMTP Port 25
Microsoft TMG has two way of networking, routing and NATing.
For this mailflow to work you need to NAT the inncomming request and not routing it.
But not everyone can change from routing, so you would need this to go outside your TMG and directly to your Exchange On-Prem Hybrid Servers.