Hi @Dan Glynn ,
First, remove the auto-mapping feature of the shared mailbox, then manually add it again in Outlook and check the result. If you are not an administrator, you will need to remove the automatic mapping of shared mailboxes with the help of a global administrator: How to remove automapping for a shared mailbox in Outlook for Microsoft 365
You can either add the shared mailbox as a separate email account in the existing Outlook profile or load the shared mailbox directly in a separate Outlook profile.
Besides, another easy way is to open the mailbox in webmail and add the rule there.
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