This guide shows you how to add a Microsoft / Office 365 shared mailbox to mac mail.
Open Mac mail and go to the top menu and select ‘Mail’ then ‘Add Account’.
Click on ‘Other Mail Account’ and choose ‘Mail account’ if asked then ‘Continue’.
Enter in the shared mailbox name and email address.
Enter your own Microsoft 365 password here, the one for your own email address. Microsoft uses the logon details of an account which has permissions to access the shared mailbox. Do not try and reset the password of the shared mailbox, just enter your own password here.
Now you should get a screen like this. The important parts to notice is the username. You must enter your own Microsoft 365 email address followed by forward slash then the first part of the shared mailbox address.
In this example the username i enter for the invoices shared mailbox is: email@example.com/invoices
Then I enter the password for my own firstname.lastname@example.org account.
Then change the incoming and outgoing mail servers as shown here:
Click ‘Sign in’ and so long as you have permissions to the shared mailbox it will work.
Tick the boxes shown and click on ‘Done’.
The shared mailbox will now being syncing to mac mail and will be visible in the left hand panel.