Adding a custom header to outgoing e-mails

In the next section, a unique header will be added to all outgoing e-mails based on which the eperi Milter app will be able to identify outbound e-mails in scope of decryption.

1. In the Microsoft 365 admin center, open the menu on the left and click Admin centers > Exchange.
   The page Exchange admin center opens in a new browser tab.
2. From the vertical menu on the left, click Mail flow > Rules.
3. Click + Add a rule > Create a new rule.
4. In the New transport rule window, enter a name for the new rule.
5. As a condition (Apply this rule if), select The recipient is > is external/internal and then select Outside the organization.
6. As an action (Do the following), select Modify the message properties > set a message header.
7. Define a unique header name and header value by clicking on Enter text.
   Note: The header name and the corresponding value can be freely chosen, but should be unique. These values are later needed for Configuring the eperi Milter app
8. Click on Next to go to the Set rule settings section. The settings can be left at the default values.
9. Click on Next to go to the Review and finish section. Review the conditions and settings of your transport rule and click on Finish to create the transport rule.
10. After the rule has been created, click on Done.
11. In the Rules overview, click on the newly created rule and enable it by toggling the Enable or disable rule switch.