Outlook 2016 Connecting to Office 365 Instead of Your Exchange Mailbox

We’ve seen situations where Outlook 2016 connects to Office 365 even when you’re trying to set up an on-premise Exchange mailbox. This happens even if Autodiscover isn’t configured to point to Office 365.

What’s causing it Recent versions of Outlook 2016 include a feature called "Direct Connect to Office 365". When it kicks in, Outlook may bypass your on-premise exchnage server and attach itself to an Office 365 mailbox.

This can happen when:

• The user has an Exchange Online license, which creates a mailbox in Office 365.
• Autodiscover isn’t responding correctly.
• A firewall or antivirus tool is blocking the connection to the on-premise environment.

If Outlook connects to the cloud mailbox by mistake, mail still appears to work, but new messages will land on the on-premise server. This can lead to sync problems, missing mail, incorrect account details, issues adding other accounts, or a temporary mailbox that doesn’t have your full data.

How to fix it You can stop Outlook from detecting Office 365 by adding a registry entry. Be careful when editing the Registry, as incorrect changes can affect system stability.

Run the following command in PowerShell:

Set-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Outlook\AutoDiscover" -Name 'ExcludeExplicitO365Endpoint' -Value 1 -Type DWORD -Force

Using Command Prompt

reg add HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeExplicitO365Endpoint /d 1

Restart the computer after running the command. The change won’t apply until the restart is complete.

Once the system is back up, create a new Outlook profile. Outlook should connect to the correct mailbox and stop redirecting to Office 365.