turn off the expand/collapse feature

Classic Outlook for Windows does not document a way to remove the expand/collapse button, but you can force message headers to remain expanded by setting the MinimalHeaderOn policy value to 0.^1^

Keep message headers expanded

Editing the registry incorrectly can affect Outlook; back up the relevant key or create a restore point first.

1. Close Outlook.
2. Press Windows+R, enter regedit, and select OK.
3. Go to: HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\Office\16.0\Outlook\Options\Mail
4. If necessary, create the missing Outlook, Options, or Mail keys.
5. Under Mail, create or edit a DWORD (32-bit) Value named: MinimalHeaderOn
6. Set its value to:
   • 0 — headers expanded by default
   • 1 — headers collapsed by default
7. Restart Outlook.

Alternatively, run this command from Command Prompt while Outlook is closed:

reg add "HKCU\SOFTWARE\Policies\Microsoft\Office\16.0\Outlook\Options\Mail" ^
 /v MinimalHeaderOn ^
 /t REG_DWORD ^
 /d 0 ^
 /f

This setting applies to the Office 16.0 generation of Classic Outlook, including Outlook 2016 and Microsoft 365 Classic Outlook. The expand/collapse control remains present; Microsoft’s interface documentation still describes using that control to open or close the message-header section.^2^

If Outlook is managed by an organization and the setting is overwritten or cannot be changed, the corresponding policy must be configured by the Microsoft 365 administrator.

References

1. You cannot edit the message subject through Outlook
2. [Classic Outlook](https://support.microsoft.com/outlook/mail/show-hide-and-view-the-bcc-blind-carbon-copy-field-in-outlook-for-windows)