Problem
You’ve used the following TechNet article to allow Exchange 2016 mailboxes to access your Exchange 2010 public folders during a migration:
Configure legacy public folders where user mailboxes are on Exchange 2013 servers
https://technet.microsoft.com/en-us/library/dn690134(v=exchg.150).aspx
You’ve confirmed that Outlook 2010 and 2013 clients are able to expand the Exchange 2010 hosted public folders but the following error messages are displayed when attempting to expand with Outlook 2016:
Cannot expand the folder. The set of folders cannot be opened. Network problems are preventing connection to Microsoft Exchange.
The set of folders cannot be opened. Network problems are preventing connection to Microsoft Exchange.
***Note that it is not accurate to test via Exchange 2016 OWA because it would not be able to proxy requests over to Exchange 2010 so you are guaranteed to receive the following error if you try opening the public folders:
Can’t complete your request
No public folders are available. Check that a public folders deployment exists in this Exchange organization. If so, please try again later
Solution
This issue had me stumped for half a day because attempting to search this error message with the combintation of Exchange 2010, 2016 and Outlook 2016 returns many other KBs and forum posts that are unrelated to the issue. After a bit more searching with different strings, I managed to find the solution in the following KB:
Outlook can't access public folders hosted on legacy Exchange servers
https://support.microsoft.com/en-us/help/3177600/outlook-can-t-access-public-folders-hosted-on-legacy-exchange-servers
Executing the following cmdlet on an Exchange 2016 hosted mailbox displays the DefaultPublicFolderMailbox field as being blank:
Get-Mailbox tluk | FL *public*
Use the following cmdlet as described in the KB to define the proxy mailbox that was created for Exchange 2016 users to access the Exchange 2010 public folders:
Set-Mailbox <User> -DefaultPublicFolderMailbox <ProxyMailbox>
Execute the following cmdlet to assign the public folder database for the user's mailbox database:
Set-MailboxDatabase (Get-Mailbox <User>).Database -PublicFolderDatabase (Get-MailboxDatabase (Get- Mailbox <ProxyMailbox>).Database).PublicFolderDatabase
Complete the above steps by executing the following cmdlet to restart the Microsoft Exchange Rpc Client Access service on the server that's running Exchange Server that hosts the public folders:
Restart-Service MSExchangeRPC
With the above completed, the Outlook 2016 client should now be able to expand the public folder structure:
