Yes, most messages protected by Microsoft Purview Message Encryption are discoverable. Microsoft Purview Message Encryption protected mail that you receive from another Microsoft 365 organization that has custom branding applied through a mail flow rule is undiscoverable by your eDiscovery service. In other words, if the mail isn't accessible through the user's mailbox, but rather, surfaced only through a link to the encrypted message portal, the mail isn't searchable. See eDiscovery activities that support encrypted items for details.