This idea has been merged into another idea. To comment or vote on this idea, please visit DOMINO-I-880 Adding a caution for inbound emails from outside organization.
Some customers asked for the possibility of adding some text in the body of an email, when the mail come from ouside the organization. Exchange/Outlook has this feature. When someone receives a mail from another company a warning is added "CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe". See here https://community.spiceworks.com/topic/2261854-caution-this-email-originated-from-outside-of-the-organization-ms-365