You have a Microsoft 365 E5 subscription.

To ensure that encrypted email messages sent to external recipients can be revoked or expire within seven days, you need to configure a sensitivity label with encryption settings in Microsoft Purview Information Protection. A sensitivity label allows you to encrypt emails and documents, set expiration policies (e.g., emails expire after 7 days), and enable email revocation

How to configure it?

● Go to Microsoft Purview compliance portal → Information Protection

● Create a sensitivity label

● Enable encryption and configure the content expiration policy

● Publish the label to users