Step 1 – Requirement

You need to configure a mail flow rule in Exchange Online to apply a Microsoft Purview Advanced Message Encryption (OME) branding template named OME1.

Step 2 – Mail Flow Rule Conditions

In Exchange Online, to trigger encryption based on sensitivity labels (classifications):

You use the condition "Apply this rule if: The sender … Includes the classification".

This lets the rule detect when the sender applies a particular sensitivity label (classification) to the email.

Step 3 – Mail Flow Rule Action

To apply a custom branding template (such as OME1), you configure the action:

"Modify the message security".

This is the action that allows applying encryption and assigning a specific OME template for branding.