Explanation:

The mail flow rule is configured to apply OMEv2 protection ("Protect with OMEv2") to messages. With OMEv2:
Recipients on consumer mail systems (such as Gmail) receive a link-protected message and must authenticate to the Office 365 Message Encryption portal to view the content.
Microsoft 365 recipients (even in external tenants) get a native reading experience in Outlook/OWA where protected messages are opened directly with automatic decryption using their Microsoft 365 identity-no separate OME portal workflow is required.
These behaviors are documented by Microsoft for OMEv2 user experiences for different recipient types and clients. See: Microsoft Purview Office 365 Message Encryption overview and user experiences for external Microsoft 365 and consumer email recipients.