I have recently migrated a customer from Exchange 2008 to 2013. Pre migration, the users who access the server remotely using Exchange over HTTP had no issues connecting to a shared calendar in public folders. Post migration, internal users who connect via the LAN can connect without issue but external users who are not joined to the domain or on the LAN cannot view the public folder Calendar.
Steps already taken...
1) Internal and External DNS and virtual directories are accessed via the same paths.
2) Logged in to an internal LAN connected domain joined PC as an external users and added the Public Calendar to the favorites list but still unavailable via OWA or Outlook over HTTPS. Public folder Calendar was accessible using this method.
3) I don't see any issues in the Outlook connection status or Email auto configuration test.
How to I troubleshoot this further?