This policy setting controls whether Outlook displays a warning before sending an item that contains an unsafe attachment ...

This policy setting controls whether Outlook displays a warning before sending an item that contains an unsafe attachment that will be blocked when the item is opened by a recipient. To protect users from viruses and other harmful files, Outlook uses two levels of security, designated Level 1 and Level 2, to restrict access to files attached to e-mail messages or other items. Outlook completely blocks access to Level 1 files by default, and requires users to save Level 2 files to disk before opening them. Potentially harmful files can be classified into these two levels by file type extension, with all other file types considered safe. If you enable this policy setting, Outlook will not display a warning when a user sends an item with a Level 1 attachment, which can cause users' data to be at risk.If you disable or do not configure this policy setting, when users attempt to send an item to which a level 1 file has been attached, Outlook warns them that the message contains a potentially unsafe attachment and that the recipient might not be able to access it. Important: This policy setting only applies if the "Outlook Security Mode" policy setting under "Microsoft Outlook 2016\Security\Security Form Settings" is configured to "Use Outlook Security Group Policy."