SOLUTION: New Outlook Mac “Cannot open this mailbox” error – Microsoft …

Select “Shared mailboxes”. In the “Search this list” box, enter the email address or name of the affected shared mailbox. Click the name of the shared mailbox in your search results. In the pane that appears at the right side of the browser, click Edit under “Email apps.” Place a checkmark in the box for “Outlook for desktop (MAPI)” ( see image …

https://answers.microsoft.com/en-us/outlook_com/forum/all/solution-new-outlook-mac-cannot-open-this-mailbox/1af6ed25-e49a-4d14-9aa1-8ae0b4e7bca7