Our email is being handled by a cloud service provider. We ran into an issue where outlook is showing (trying to connect) . Meanwhile, OWA is working fine. OWA and Outlook have the Same host public ip.
We are using office 2016 package as MS. We don't have 365.
Exchange server is 2019.
Outlook is not connecting from internal network only.
-When accessing from internal network to url >> Https://(server name) /ews/[login to view URL] there is an error showing (service unavailable 503). However, when accessing the same url on personal network (external network) the url is prompting username /password and no error displayed.
Resolution was charging our public ip from our external FW and it is from the same range of the previous public ip configured. Moreover, after outlook is connected with the new public ip, we shifted back to our old public and it worked fine.
My question is what was the root cause of the problem. (exchange server, email provider network, our internal network, our internet service provider)
--------Troubleshootings and Findings------
Network : no disruption whatsoever on any other services at all. All needed ports are allowed and no changes occurred from network side.
Local DNS: as per the email service provider requirements (entery for autodiscover only)
Public DNS: mx records are accurate.
Troubleshooting Done :
- Tracert to email provider public ip is reaching.
-Nslookup is accurately resolved.
-Connection status from Outlook application showing (Auth error).
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I am working with Microsoft. it seems you are looking the RCA of the issue. I work daily on such cases, feel free to ping me to discuss it further