Outlook 2016 Prompting for Credentials on Mailboxes Users Have Full Access To

A user reported that they are receiving multiple credential prompts on their Windows 10 PC in Outlook 2016 for other user mailboxes which he has full access to within Exchange and they are configured to auto mount. Another user in the organization with Outlook 2016 is also configured with full access to one of these mailboxes and does not receive any prompts for it. I began to suspect it was something within his Outlook profile or his mailbox however after making a test user and assigning full access permissions to one of these mailboxes I am seeing the same behavior in Outlook 2016 from my PC.

Troubleshooting
1. Ran the Outlook Autodiscover connectivity analyzer test and it came back healthy.
2. Cleared the user's credential manager on the PC
3. Putting in the correct username/password for the user mailbox they have full access does not stop the prompt even after confirming the credentials work in OWA
4. Confirmed he was not manually mapping the mailboxes in the Outlook profile configuration
5. Ran an online repair of his office suite.
6. Created a test user account and assigned full control to one of the mailboxes giving him the prompt for credentials and I experience the same behavior.
7. Toggled full access permissions off and back on
8. Disabled and reenabled inheritable permissions in AD
9. The user that reported the problem also has a Surface Pro with Outlook 2013 and he does not get the credential prompts on the Surface.
10. Even though the prompts occur and continually occur he is able to access the mailboxes and see new emails.

Any assistance is much appreciated.
PCNS_TechAsked:
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FOXActive Directory/Exchange EngineerCommented:
Remove the outlook profile then launch outlook in safe mode(click start, run, type in    outlook /safe  (press Enter)) let it build a new profile.  When the login pops up put in the credentials in again to see if it takes.
PCNS_TechAuthor Commented:
Fox,

I gave that a shot but no dice the prompt still occurs and when entering the credentials for it it continually pops up . My bigger question is why it would ever even prompt for credentials to this mailbox since the account the Outlook profile is configured for has full access to it. Appreciate the help.

Thanks,
Jesse
Olgierd UngehojerSenior Network AdministratorCommented:
You have to add priority to use autodivovery from your domain zone first not from office365. This  new think because Microsoft change this. You have to do this in the windows register. It happen after last office update.
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PCNS_TechAuthor Commented:
Disabling automount for the full access permissions seems to have alleviate the problem. After mounting manually in the Outlook profile the issue has stopped.

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DonNetwork AdministratorCommented:
I'm afraid that don't work as I am experiencing the same issue


[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover]
"ExcludeHttpsRootDomain"=dword:00000001

does not help
Olgierd UngehojerSenior Network AdministratorCommented:
In my case it was add a registry key that "skips" the O365 account looking _first_:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
and add the following:
DWORD32: ExcludeExplicitO365Endpoint
Value=1
Seth SimmonsSr. Systems AdministratorCommented:
No comment has been added to this question in more than 21 days, so it is now classified as abandoned.

I have recommended this question be closed as follows:

Accept: PCNS_Tech (https:#a42436333)

If you feel this question should be closed differently, post an objection and the moderators will review all objections and close it as they feel fit. If no one objects, this question will be closed automatically the way described above.

seth2740
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