Hi,
I'm in the process of migrating to a hybrid O365 solution but having some challenges with outbound mail from On-prem mailbox to mailboxes in Exchange online. What we're seeing is Exchange is trying make a connection with smart host tenent.mail.protection.onmicrosoft.com but is failing. My question is, how does Exchange make decisions about how to route traffic? I'm not talking here about connectors which manage point to point mail flow. I'm more interested in traffic at the network level.
We use a proxy server in our environment and I've defined this in -internetwebproxy (set-exchangeserver). I can see that traffic to port 80/443 is using the proxy. Less clear is why traffic to Exchange Online over port 25 is trying to go direct to the internet. The EOL send connector is configured to "used DNS (MX) records to route mail automatically". As a result the connector tries to connect to a smart host which is the IP associated withtenent.mail.protection.onmicrosoft.com over port 25. The problem is, instead of going out over the proxy, traffic is trying to go direct and is being blocked by our firewall.
So to summarise, how does Exchange make decisions about how network traffic should flow?
Thanks in advance