In Microsoft Outlook 2016 for Mac and Outlook for Mac 2011, you cannot connect to legacy public folders. These are public folders stored on a version of Exchange that is earlier than the version on which your current mailbox is stored.
For example, if your mailbox is homed on Microsoft Exchange Server 2013, you will be unable to connect to public folders stored on Microsoft Exchange Server 2010 or Microsoft Exchange Server 2007.
This is a limitation of the EWS protocol that Outlook for Mac uses to connect to Exchange Server.
To resolve this issue in Outlook 2016 for Mac with Exchange Server 2016 or Exchange Server 2013, install the following updates:
On the Outlook 2016 for Mac client: Install the April 12, 2016, update for Outlook 2016 for Mac (version 15.21.1), or a later update. For information about this update, click the following article number to view the article in the Microsoft Knowledge Base:
3142577 MS16-042: Description of the security update for Office 2016 for Mac: April 12, 2016
On the Exchange Server: Install Cumulative Update 2 for Exchange 2016, or Cumulative Update 13 for Exchange Server 2013, or a later cumulative update. For information about these updates, click the following article number to view the article in the Microsoft Knowledge Base:
If you can't install the updates that are mentioned in the "Resolution" section to fix this issue, to work around this issue, migrate the legacy public folders to the current version of Exchange. Or, move the mailbox back to the earlier version of Exchange on which your public folders are currently stored.