Forum Discussion
FIX - Outlook 2013,2016,2019 fails open mailbox Exchange 2019 on-prem in offline LAN
Exchange 2019 on-prem + Outlook 2013/2016/2019 in offline LAN
Symptoms:
- OWA works
- ECP works
- Autodiscover works
- Test-MAPIConnectivity is successful
- Outlook profile can be created
- Outlook fails to open the mailbox / “Cannot start Microsoft Outlook” / “The set of folders cannot be opened” / “The attempt to log on to Microsoft Exchange has failed”
- Environment has no internet connection
Root cause:
The Windows client had a default gateway configured, but the gateway IP did not respond to ping. In our case the client received 192.168.1.1 as default gateway, but this IP was unreachable in the offline network.
Fix:
Set the client default gateway to an existing reachable IP address, for example the Exchange/DC server IP 192.168.1.5. Internet access is not required, but the default gateway must be reachable/responding.
After changing:
Default gateway: 192.168.1.5
DNS: 192.168.1.5
mail/autodiscover DNS or hosts pointing to Exchange 2019
Result:
Outlook 2013, Outlook 2016 and Outlook 2019 connected to Exchange 2019 successfully.
1 Reply
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?
