Hi Sean,
I am sorry for the delayed answer. To troubleshoot the issue, please follow the steps below to perform a test of Outlook Anywhere.
1. Click on the link https://www.testexchangeconnectivity.com/
2. Click on Office 365 tab. Select Outlook Anywhere (RPC over HTTP) under Microsoft Office Outlook Connectivity Tests.
3. Click Next and fill out the required information. Click Perform Test to start the test. Please make sure type in the affected user’s email address in the Email address text bar and Microsoft Account.
Please capture a screenshot of the test result and post it here for troubleshooting.
Thanks,
Yang Yu