UPDATE 8/2/23: Another way to accomplish this is to enable the built-in external tags now supported by M365 Apps (Build 2211 and greater), Outlook on the Web, and Outlook Mobile. Note that this external tag will not work in older versions of Outlook or M365 Apps (prior to build 2211), so you may still need the original solution listed in this article.
If you are on M365 Apps, you can connect to Exchange Online PowerShell and run the following command: Set-ExternalInOutlook -Enabled $true
For more information, check Microsoft’s article Native external sender callouts on email in Outlook
Adding an external sender notification to the top of an email is an important distinction for many companies. This disclaimer quickly identifies its end users when a message is sourced from an external sender. This eliminates the guesswork for internal users, helping them to identify potential phishing attacks, but also a great reminder when it comes to data loss prevention as they reply.
Companies approach this disclaimer in many different ways. Two common examples are a disclaimer prepended at the top of the email or adding a keyword in the message subject.
Thankfully, adding this is a simple process in Office 365 (and also Exchange on-premises – the instructions are identical).
For this article, our example company, Time Travel Research, wishes that all inbound email from external senders is prepended with a disclaimer stating the sender is external to the organization. Time Travel Research wants to ensure that every instance of an external email, even those in the same email chain, is prepended with this disclaimer.
Let’s get started!
Add an external sender disclaimer to all inbound email
Log in to the Exchange Admin Center. Once logged in, navigate to Mail Flow >> Rules. Click the New (
) button.
From the drop-down menu, you will notice several choices. These choices are predefined rule templates. We will create a rule from scratch. Select Create a new rule.
