Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
Exit all Microsoft Office applications.
Start Registry Editor:
In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
In Windows 8 or Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
In Windows 7 or Windows Vista, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
Locate and then select the following registry subkey:
Outlook 2013 HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences\
Outlook 2016 HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences\
On the Edit menu, point to New, and then click DWORD Value.
Type ForceWarnAboutExternalContent, and then press Enter.
In the Details pane, right-click ForceWarnAboutExternalContent, and then click Modify.
In the Value data box, type 1, and then click OK.
Note After you set the registry key, you can disable and enable the Warn me before downloading content when editing, forwarding, or replying to e-mail option in trust center.