Outlook Integration displays "The email server is unavailable" in shared mailbox

If you’re a member of the shared folder with full access and make sure that auto-mapping is enabled for your email account.

An Exchange Server feature known as "automapping" is on by default which means that subsequently the shared mailbox should automatically appear in a user's Outlook app after Outlook has been closed and reopened. They also suggests Outlook add-ins may work when automapping is off, and provided steps mentioned in Open and use a shared mailbox in Outlook on how to add a shared mailbox as an additional mailbox to an existing user account in Outlook.

NOTE: The same setup works fine in OWA and New Outlook. Microsoft should be better positioned to address an add-in issue where it does not work in classic Outlook desktop app.