Local DNS after Office 365 migration

Outlook, uses "Autodiscover" to find out where the user mailbox is, as long as you have Exchange server running in your network, the users will always be pointed to that server before anything.

There are different stages for autodiscover, listed here according to the order Outlook uses them:

1. SCP lookup: Exchange will save some helpful information for Outlook inside Active Directory, Outlook will query that information to find out where the user mailbox is.
2. HTTPS root domain query: Outlook will run a query to the domain name the user is using, for exmaple: https://testdomain.com/autodiscover/autodiscover.xml
3. HTTPS Autodiscover domain query: Outlook will add the domain "autodiscover" to the HTTPS root domain query: https://autodiscover.testdomain.com/autodiscover/autodiscover.xml.
4. HTTP redirect method for any of the links above.
5. SRV record query stored in DNS.
6. Local XML file configured by the admin.
7. Cached URL in the Outlook profile.

I would start with the SCP lookup method, as usually this is the default method for Outlook to find its mailbox inside a domain, you will need to disable that using the following steps:

• Open Exchange Management Shell "EMS"
• Run the command "Get-ClientAccessServer": this will list the Client Access Server "CAS" configured in your domain.
• Run the command "Set-ClientAccessServer -Identity "CASName" -AutoDiscoverServiceInternalUri $NULL" to clear the SCP lookup information from the domain, where CASName is the result of the Get-ClientAccessServer command.

IMPORTANT NOTE: running this command will disable user access to the internal mailbox, you will need to configure each user who have their mailbox still hosted in the internal Exchange server manually.