Outlook 2016 and Exchange 2010 Public Folder Woes

Upon finishing my last project and stopping to take a look back at the past year or so I’ve realized that most (90%+) involved Office 365. Quite a dramatic change from my previous rate of around 50%. I have worked on a couple of migrations over the past year that revealed that Outlook 2016 is quite a different animal than all of its predecessors. Specifically when it comes to Public Folders on Legacy Exchange 2010 servers which is the last Exchange Server version to support Public Folders with actual Public Folder databases.
The Problem
In my most recent migration I had a customer that was moving from Exchange 2010 to Exchange 2016. In doing to we needed to configure coexistence for Public Folders. The reason is that with Public Folders, if a mailbox is on Exchange 2016, it can access Public Folders on Exchange 2016 AND Exchange 2010. However, a user with a mailbox on Exchange 2010 can only access Public Folders on Exchange 2010. This is due to the way that Microsoft has always proxied requests from a higher version to a lower version, but never in the reverse (exception now would be 2013/2016).

With this particular migration we configured the coexistence per Microsoft’s own documentation:
https://technet.microsoft.com/en-us/library/dn690134(v=exchg.150).aspx
The client had two Exchange 2010 Servers with Public Folder database replicas on both servers. This means two new mailboxes to serve as proxies and two new mailbox databases according to Microsoft. We set this up and it appeared to work for IT (Outlook 2016) and myself (Outlook 2013). We gave the testing a green light and proceeded to move a lot of mailboxes. When these users moved to Exchange 2016 we started getting weird reports of Public Folders not expanding.
Troubleshooting
To resolve the issue, we performed the following steps:

• Gathered up names of users with issues.
• verified the ‘EffectivePublicFolderMailbox’ property on the user’s mailbox
• Reviewed the Exchange Organization Config with ‘Get-OrganizationConfig’ to verify each Public Folder Proxy Mailbox was configured.
• Verified mailbox database for Public Folder mailbox was mounted.
• Verified Outlook up to date
• Tried a new profile – same results

After all of this it appeared that everything was properly configured. After a bit of research online about this I was able to dig up a KB article from Microsoft Explaining the issue.
https://support.microsoft.com/en-us/help/3177600/outlook-can-t-access-public-folders-hosted-on-legacy-exchange-servers
With this article in hand we checked the DefaultPublicFolderMailbox setting to see which proxy mailbox was being used. We then checked the users database ‘Public Folder Database’ property and compared that against the one for the Public Folder Proxy mailbox. What we found out was there was a mismatch. with over 1000+ mailboxes, we determined around half were configured incorrect according to Microsoft. In order to fix this, we simply made a system of matching the same Public Folder database between the one the user was pointed to and the one the Public Folder mailbox’s database was pointed to.
Here is how it looks visually:
To fix the issue, we need the everything to line up like so:
We need the mailbox for the user and the mailbox for the Proxy Public Folder mailbox (which is a property of the mailbox) to point to the same Public Folder Database.
Using PowerShell for Discovery
We can use PowerShell to discover what doesn’t match. In the sample below, I put in the known values of what the Public Folder Proxy mailbox should be (according to the database the mailbox is in) and then I compare that versus what is set in the ‘Effective Public Folder Mailbox’ attribute on the users mailbox. If these don’t match, I report the values back in yellow for emphasis and if they are correct, I can just report it back as regular white text.

$Servers=@()
$Servers = 'Server1','Server2'
Foreach ($Server in $Servers) {
    $Mailboxes = Get-Mailbox -server $Server
    Foreach ($Mailbox in $Mailboxes) {
        $Name = $Mailbox.name
        $EPFM = $Mailbox.EffectivePublicFolderMailbox
        $Database = $Mailbox.database
        $PFDB = (Get-MailboxDatabase).PublicFolderDatabase
        $PFMBX = $EPFM -split '/'
        $UsersPublicFolderMailbox = $PFMBX[2]
	If ($Database -eq 'DB1') {
		If ($EPFM -notlike '*PFProxyMbx01') {
		        Write-host "$Name,$UsersPublicFolderMailbox,PFProxyMbx01,$Database" -ForegroundColor Yellow
		} Else {
			Write-host "$Name,$UsersPublicFolderMailbox,PFProxyMbx01"
		}
	}
	If ($Database -eq 'DB2') {
		If ($EPFM -notlike '*PFProxyMbx02') {
		        Write-host "$Name,$UsersPublicFolderMailbox,PFProxyMbx02,$Database" -ForegroundColor Yellow
		} Else {
			Write-host "$Name,$UsersPublicFolderMailbox,PFProxyMbx02"
		}
	}
    }
}

This script can produce a list of users affected and needed to be fixed. IT simply checks to make sure that the values match as expected:

If I need to correct this, I can use a script like the one below with some pre-populated values as well:

Get-Mailbox -Database USDB05 | Set-Mailbox -DefaultPublicFolderMailbox PFProxyMbx01

This snippet of code makes sure that all mailboxes in this mailbox database get the correct Public Folder mailbox. For migrations, we can pre-create scripts like this to make sure all the mailboxes, even newly moved ones, have the correct Public Folder Proxy mailbox.
Hope this code and article help you out in your migrations to Exchange 2016.
** Note ** As a side note, this is also effective or useful for Exchange Online moves as well. The same configuration and alignment of Public Folder proxy mailboxes apply the same way and can cause issues with users if not configured the same way.