Stop Outlook 2016 from using Office 365 autodiscover
A single registry value, ExcludeExplicitO365Endpoint, stops Outlook 2016 from going straight to Office 365 during autodiscover, so it uses your own autodiscover records instead.
Infrastructure microsoftContents
During autodiscover, Outlook 2016 first tries to reach Office 365 directly, before it looks at your own autodiscover records. In a hybrid or on-premises setup this can connect users to the wrong place. Microsoft describes the behaviour in Unexpected autodiscover behavior when you have registry settings under the AutoDiscover key.
Disable the Office 365 endpoint
Run this in a Command Prompt as the user:
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeExplicitO365Endpoint /d 1With this value set, Outlook skips the direct Office 365 check and falls back to the other autodiscover methods, such as your DNS records. Your mailbox and other Office 365 services keep working; only the way Outlook finds the server changes.
Before you roll it out
Changing the registry needs care. Back it up first, and test the change outside production, especially if you plan to deploy it with an automation tool: excluding the Office 365 endpoint can cause unexpected behaviour with mailboxes that are already in Exchange Online.