I know this is a lot of info but I have been banging my head against my desk all week trying to understand this.This solution Details how to correctly manually add a Shared Mailbox to the Outlook Client on both Windows and MAC. I am looking into Azure Information Protection client - but I'm not sure if that will help. Lastly, I also removed the user from the shared mailbox and re-added them with automapping enabled via PowerShell. I also tried adding a shared folder in the users OWA to the shared mailbox, but still no luck. The same issue happens when Edge launches. I created a profile for the shared mailbox and opened Outlook with that profile.

Workflow wise, is there a way to open this message in Outlook, or does the user have to use OWA from the shared mailbox? The error only happens when they access the shared mailbox from the Outlook desktop client. If the user opens the Shared Mailbox in OWA, they can see the message no problem. It appears that the shared mailbox has permissions to see the contents but when Edge is opened after clicking 'Read the Message' in Outlook, the authentication defaults to the users account instead of the shared Mailbox. After a while an error in Edge appears 'This Message might have been moved or deleted' When they click the read message link, Microsoft Edge Launches and tries to open the message. The user states they were able to open the messages before but something has changed. I'm not sure if this related to the service alert regard encrypted messages.

I have a user who has full access with auto mapping to a Shared mailbox who cannot open protected messages.

Open shared inbox outlook