Explanation:
Box 1: A mail flow rule in Exchange admin center
To automatically encrypt all emails sent to a specific domain with Microsoft Purview Advanced Message Encryption, an administrator must first configure a sensitivity label with the desired encryption settings and then create an Exchange mail flow rule (also called a transport rule) in the Microsoft Purview portal to apply that sensitivity label to messages meeting the condition of being sent to the target domain.
Box 2: A custom branded template in Microsoft Exchange Online PowerShell You can use message expiration on emails that your users send to external recipients who use the OME Portal to access encrypted emails. You force recipients to use the OME portal to view and reply to encrypted emails sent by your organization by using a *custom branded template* that specifies an expiration date in PowerShell.
Reference:
https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/manage- mail-flow-rules
https://learn.microsoft.com/en-us/purview/ome-advanced-expiration