Microsoft Office 365 allows administrators to create custom mail rules to perform a variety of tasks. In this tutorial, we'll create a rule that clearly marks external emails - those emails coming from outside of the organization - as "External" to alert mail users to be cautious with the contents of the message.

1. Go to the Exchange Admin center in your Office 365 Subscription

2. Under mail flow, click the “+” icon to add a new rule. Select “Modify messages...”

3. In the “New Rule” screen, we want to prepend the word “[EXTERNAL]” to the subject line. Use the settings from the screenshot below.

4. Save the rule and you’re done! Now all email coming from outside of your organization will be clearly identified as coming from an external source.

5. Done