Public Folder Proxy Process

Many businesses still have large volumes of data stored in public folders. When considering a move to Office 365, how to address public folders requires some thought.

Firstly, public folders come in two flavours, legacy are those hosted on Exchange 2007 or 2010 and modern are those hosted on Exchange 2013 or 2016. This distinction arises due to the change in architecture in providing public folder functionality.

Public folders can be migrated directly to Exchange Online, and Microsoft has created the following scripts to help you get them there https://www.microsoft.com/en-us/download/details.aspx?id=38407 for legacy and https://www.microsoft.com/en-us/download/details.aspx?id=54855 for modern.

However, if you go ahead and migrate your public folders straight to Exchange Online, your users left behind on the on-premises infrastructure (either because that is where they will stay, or they haven’t yet been migrated) will not be able to access them.

So how do you provide access to public folders to users hosted both in Exchange Online and on-premises?

Legacy On-premises Public Folders or Public Folder Proxy is the answer.

The method for configuring access to legacy public folders is well documented elsewhere, for example https://www.microsoft.com/en-us/download/details.aspx?id=54855, the purpose of this post is to give an overview of how this process works from the client side.

The diagram below shows how this looks for a mailbox hosted in Exchange Online.

PF Proxy Process

  1. The user access their mailbox which is hosted in Office 365
  2. During the initial Autodiscover process, as Exchange is configured for Remote public folders, it returns the SMTP address of a proxy mailbox to use. The proxy mailbox associated to the  user can be viewed via Get-Mailbox | EffectivePublicFolderMailbox
  3. Outlook uses this SMTP address to perform a secondary Autodiscover lookup. This resolves to the on-premises Exchange environment
  4. The XML data of URLs and connection information for the proxy mailbox is returned to the client
  5. Outlook uses the proxy mailbox
  6. to connect to the public folder database associated with the mailbox database holding the proxy mailbox
  7. Public folder information is returned to the Outlook client.

For mailboxes still hosted on-premises access remains the same as it was, via native connection methods.

Once all mailboxes are moved to Exchange Online, the opportunity to also migrate the public folders arises, but of course some due diligence should be carried out to ensure there are no other dependencies on public folders on-premises.

So, this provides the solution for the co-existence phase of the move to Exchange Online, and gives some breathing space to consolidate some of the potentially stale data still hosted in public folders before the move.

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