When having a local setup of Exchange and you want to migrate to Office 365 while leaving the local Exchange in place, you will have problems with autodiscover still pointing users to the local Exchange setup. If you don’t do the below, Outlook will still try to connect to the old Exchange server.

To stop the Autodiscover, open ADSIEDIT.MSC from the Active Directory server and delete the below entry so that the local SCP entry is skipped.

Select the “Configuration” naming context
CN=Services\
CN=Microsoft Exchange\
CN=\
CN=Administrative Groups\
CN=Exchange Administrative Groups\
CN=Servers\
CN=\
CN=Protcols\
CN=Autodiscover\
And delete the CN= of class serviceConnectionPoint

Since Outlook uses SCP as well, you might want to run this registry update on the local machines.

(The version number varies depending on your Office application version)
– Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
– Create new DWord ExcludeScpLookup
– Set the DWord as 1

After this, try to ping autodiscover.mydomain.com to ensure that it’s pointing to the Office 365

Update:

You can also do the following maybe it will help more. These should be created under the AutoDiscover Key

“ExcludeScpLookup”=dword:00000001
“ExcludeHttpsAutodiscoverDomain”=dword:00000001
“ExcludeHttpsRootDomain”=dword:00000001
“ExcludeSrvLookup”=dword:00000001
“ExcludeHttpRedirect”=dword:00000000
“ExcludeSrvRecord”=dword:00000001