FIX - Outlook 2013,2016,2019 fails open mailbox Exchange 2019 on-prem in offline LAN

Hi MaVy,

That's an interesting find. Most people would probably focus on DNS, Autodiscover, or Exchange configuration first, so I wouldn't immediately suspect an unreachable default gateway in an isolated network.

Thanks for sharing the root cause and resolution. I'm sure this will save someone a lot of troubleshooting time if they run into a similar Outlook-to-Exchange connectivity issue in an offline environment.

Out of curiosity, did Outlook generate any specific errors in the Windows Event Logs that pointed you toward the gateway issue, or was it mostly a process of elimination?