As perhttps://learn.microsoft.com/en-us/copilot/microsoft-365/microsoft-365-copilot-requirements
Microsoft Outlook
Microsoft 365 Copilot works with classic Outlook and new Outlook (for Windows and Mac). Users can switch to the new Outlook by selecting Try the new Outlook in their existing Outlook client.
Important
Microsoft 365 Copilot is only supported on primary mailboxes that are hosted on Exchange Online. It is not available on a user's archive mailbox, group mailboxes, or shared and delegate mailboxes that they have access to.
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hth
Marcin