User is prompted for credentials after a mailbox is moved from legacy Dedicated to Exchange vNext


After a user's mailbox is moved to Microsoft Office 365 Dedicated/ITAR (vNext), the user is prompted to authenticate or for credentials to access Exchange vNext. Some users are prompted more than one time.


The user may experience these symptoms depending on the client version, vNext tenant, or Autodiscover configuration. These incidents should be investigated during the pilot phase of the Exchange vNext transition.


IT administrators should be familiar with the transition guidance that's available on the Technical Scenarios Matrix ( on the EXO-vNext Transition tab. Known issues for customers transitioning to vNext are documented there.  

It's important that users run the most recent version of their Outlook client for full functionality and correct behavior. For information about the minimum required versions of Outlook in an Office 365 environment, see Outlook Updates.

For information about monthly updates and the corresponding build numbers, see Outlook and Outlook for Mac: Update file versions.

If additional investigation and collaboration with Microsoft is required, collect the following details from pilot users:
  • Approximate number of times a user was prompted for credentials
  • Approximate number of users who report this issue
  • After the user enters their credentials, can they successfully connect to Exchange by using their original profile?
  • Is the user prompted after the client first restarts, or do they continue to be prompted intermittently? 
Client diagnostic information is important to collect from these pilot users. This helps determine the cause and the changes that may be required to improve the experience for other migrated users.

Office Configuration Analyzer Tool (OffCAT)

OffCAT will provide details about the client version and Outlook-related registry keys. Common issues are related to old client versions and Autodiscover. IT administrators should review the issues list to help troubleshoot the cause of authentication prompts. If no related issues are discovered, the report should be sent to Microsoft for further analysis as part of the collaborative investigation.

A screenshot of the OffCAT page

Outlook troubleshooting logs

If users continue to be prompted intermittently for authentication and credentials, Outlook troubleshooting logging should be enabled. These details are important for any Microsoft investigation of this issue.

If users are no longer prompted for their credentials, the next group of pilot users should be contacted to proactively enable Outlook troubleshooting logging before their migration. An OffCAT report and the ETL files from the %temp%\Outlook logging folder should be provided to Microsoft for the collaborative investigation.

Fiddler traces

Depending on the information obtained from the OffCAT and Outlook troubleshooting logs, additional diagnostic information may be requested. This may include a fiddler trace. If this is required, Microsoft will send a diagnostic package to let users securely capture and upload the results of the trace.  

Article ID: 3187952 - Last Review: Aug 29, 2016 - Revision: 1