hi we have a few users getting prompted for account credentials when using standard version of outlook . office ( VLSC version)
they are using on-prem exchange server (2019)
Office version standard 2019
we recently added their domain into an M365 tenant account and configured azure ad connect to sync some users to tenant but outook and exchange server still remain on-premise.
why would outlook be prompting for Azure account information in this scenario ?
scratching our heads.
Hi @JasonGodfrey-5248,
To bettere help you, I would add the tag "office-exchange-server-connectivity".
Thanks for your understanding.
A password prompt can be seen in following situations
A user’s mailbox is on-premises, and they have access to another user or shared mailbox which has already been moved to Exchange Online
A remote archive has been created for the user
Read more details and how to resolve this from here: outlook-prompt-password-modern-authentication-enabled
These do not apply.
We have 0 mailbox moved to exchange online
the only steps that were done was the following
1 - add customer domain to o365 tenant account
2- install azure ad connect on customer premise but we only did 1 sync with 1 test OU
This is all!!!
Now users are suddenly receiving popups when using Outlook for the Azure username/password
Why is this happening ?
It could be that, while you were performing the item 2, 'Exchange Hybrid Deployment' option was selected. 
If that's the case, the process will written back certain 0365 attributes back to the on premises. So, even though you haven't migrated the mailboxes, there are some connections
yes we did check this box.
what is best practice ?
Should I not have checked the box until we awere ready to migrate ?
should i not have checked the box until ha have ad accounts sync's to Azure ?
What do you suggest for next time ?
If your are not having any users in 365 at present, I don't see a requirement to select the option. But when you start adding any mailboxes in office 365, this is a must have option. The legacyExchangeDN of a moved mailbox is written back as an X500 address on-premises to the remote mailbox to avoid any mail delivery issues with cached addresses in outlook client.
Have you added this value "ExcludeExplicitO365Endpoint" to the registry on the affected Outlook clients?
If haven't yet, please try adding it and see if it can help with this issue.
The path in registry is HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\AutoDiscover (if the Autodiscover key doesn't exist, you can manually create it)
Please create a DWORD value named ExcludeExplicitO365Endpoint and set its value to 1.
Then restart Outlook and see if it can connect successfully
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@KaelYao-MSFT is there official documentation about this ?
what is this actually doing ? why are we doing it ?
Is this specifically to address the credential prompt issue when selecting "Exchange Hybrid Deployment " when installing Azure Ad COnnect?
is there official documentation about this ?
Yes. Please refer to this link: Unexpected Autodiscover behavior when you have registry settings under the \Autodiscover key
what is this actually doing ? why are we doing it ? Is this specifically to address the credential prompt issue when selecting "Exchange Hybrid Deployment " when installing Azure Ad COnnect?
It is used to disable Outlook from trying to connect to O365 first.
Since currently Outlook is prompting for O365 credentials, which is not the expected behavior.
I am writing here to confirm with you how thing going now?
Did the issue get resolved?
Please let us know if you would like further assistance.
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