How to disable Outlook security warning when opening attachments sent by internal Exchange/organization users?

Question

Whenever our Exchange users (using Outlook 2013) try to open an attachment in their mail which has been sent by a user in the organization a security popup appears telling them that files from the internet may be harmful.

The user has to press next to open the file.

is there a way to disable this for users sending from the internal organization? I'm sure it should be possible using group policies but I haven't found a way how to accomplish this.

The internal domain has been added to the intranet zone mapping using GPO.

Help would be much appreciated!

Solution

Set the MarkInternalAsUnsafe DWORD value to 1 under HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Security

Please refer to the blog below:

https://support.microsoft.com/en-us/help/2714439/office-document-attachments-open-in-protected-view-in-outlook

https://www.msoutlook.info/question/884

Hope it helps.