Hi MH09,
In order to troubleshoot the issue, I would like to collect some information from you.
1. Do you encounter the same problem in Outlook?
2. Do you have the test via Test Connectivity Analyzer, and which test you've done, Outlook anywhere or Outlook auto-discover? If you haven’t done a test for Outlook anywhere (RPC over HTTP), please perform a test and post a screenshot here if there is any error.And try the suggestions in my previous reply.
Moreover, as a similar issue may have different root causes, to avoid the confusion, I suggest you post the information of the test result in a new thread. After doing that, we can focus on the issue form your side.
Thanks,
Edmond Peng