Migrate Public Folders to Office 365 Shared Mailbox

An Office 365 Shared Mailbox is the closest match to Public Folders as you can maintain any part of the public folder hierarchy, items, and permissions. Separate credentials are not required for a Shared Mailbox as these only need to be assigned. This makes it an attractive option for Public Folder content. When wanting to Migrate Public Folders to Office 365 Shared Mailboxes, it’s important to understand how your Public Folder(s) is being used. In this post, we’ll analyze the key factors that make Office 365 Shared Mailboxes a great match for Public Folder content and how you can plan a migration.

Understand Business Process behind Public Folders

Public Folder hierarchies from one instance to another can look similar. However, how they are utilized can be very different. The reason for this comes down the role of Public Folders within one or more business processes. Here are some common examples of Public Folders being used for a business process:

  • Receive and organize Customer communications that trigger a specific follow-up
  • File specific communications from certain users into a central location
  • Share team-critical information in a central location for easy access
  • Share one or more calendars for tracking order fulfillment
  • Archive messages

What Makes an Office 365 Shared Mailbox a Good Option for Public Folders

Certain parts of your Public Folder Hierarchy will be good candidates for migration to an Office 365 Shared Mailbox. It’s important to know the primary characteristics that can make portions of your Public Folder Hierarchy a good candidate for a Shared Mailbox:

  1. It can easily be split into one or more Shared Mailboxes
    In this case, your have a Public Folder hierarchy that is structured in such a way that you can easily identify which parts of the hierarchy should belong to its own Shared Mailbox. For example, assign each root Public Folders or a set of root and subfolders matching a department or team to is own Shared Mailbox.
  2. The Public Folder is mail-enabled
    Mail-Enabled Public Folders are assigned an SMTP address and can receive mail just like a regular user mailbox. Since the business process already relies on that SMTP address, you can easily transition it over to a Shared Mailbox and migrate the associated Public Folder content to that mailbox. It is also possible to migrate more than one mail-enabled Public Folder to a single Shared Mailbox, but we will discuss that in a separate post.
  3. You are using Calendars in Public Folders
    Certain Public Folders can be designated as Calendar folders. These are folders that show a Calendar instead of an item list view. It is the same type of calendar that you see in an individual user mailbox. Calendar folders can also be mail-enabled as well if they are used in conjunction with an Auto Reply rule, but don’t need to be in order to be a good candidate for migration to an Office 365 Shared Mailbox.
  4. You are using Contacts in Public Folders
    Much like Calendar Public Public Folders above, folders can also be designated as Contact folders. The Contact folder shows the same type of contacts list that you see in an individual user mailbox.


What Makes an Office 365 Shared Mailbox a Less than Ideal Option for Public Folders

Just as you can make the case for some Public Folders being a good fit for Office 365 Shared Mailboxes, there are also instances where it might be less than ideal. This is where understanding the business process in the Public Folder branch in question comes into play:

  1. There is no rhyme or reason to the Public Folder hierarchy
    Without knowing how to parse the Public Folder hierarchy, it can be much more difficult to determine which Public Folders should go a Shared Mailbox. This might not be a problem if have less than 50 GB of Public Folder content as you can fit it all in a single Shared Mailbox.
  2. Content in Public Folders is stored as if it were a file share
    Organizations with a long-time use of Public Folders often have portions of their Public Folders that look like a file share. Business processes are the most likely cause of this. If you can easily identify and split your Public Folders based on department or some other grouping, then this is not too much of an issue. Each Shared Mailbox is limited to 50 GB (100 GB if you purchase a license for the Shared Mailbox).

Prepare for Migration

Below is a list of tasks that should be completed prior to beginning your migration from Public Folders to Shared Mailboxes:
  • Identify the number of Public Folders and the total Public Folder size. The following PowerShell cmdlet will produce this information and the total size of each folder in MB and output to a CSV file:
    Get-PublicFolderStatistics -ResultSize Unlimited | Select-Object Name, FolderPath, ItemCount, @{Name="TotalItemSizeMB"; Expression={[math]::Round(($_.TotalItemSize.ToString().Split("(")[1].Split(" ")[0].Replace(",","")/1MB),0)}} | Export-CSV .\PF-Sizes-In-MB.cs
  • Search for any Mail-Enabled Public Folders and export their configuration. 
  • Determine the number of Shared Mailboxes required by assigning one or more Public Folders to a Shared Mailbox.
  • Build a user access list for each Shared Mailbox so that access permissions to the Shared Mailboxes can be assigned.

When to Account for Public Folder Permissions

For a user to get access to a Shared Mailbox they need two things:

  1. Send-As Permissions
  2. Full Access to the Mailbox
At a minimum, Send-As permissions are required in order to see the mailbox from Outlook. Full Access allows you to interact with the entire mailbox just like you would your own mailbox. If you want to control access to the specific folders in the Shared Mailbox then you can achieve this by assigning permissions to the specific folders. In this case you do not need to assign Full Access to the mailbox, so when a user looks at the Shared Mailbox, they will only see the folders they have explicit permissions to view.

Mail-Enabled Public Folders and Shared Mailboxes

If one or more of your Shared Mailboxes will be inheriting email addresses from a Mail-Enabled Public Folder, then preparing for the following in advance will help:

  • Know each SMTP address that needs to be moved from the Mail-Enabled Public Folder. A Mail-Enabled Public Folder can have one or more SMTP addresses assigned to it.
  • Document which SMTP address is the Primary in the Mail-Enabled Public Folder
  • Check to see if there are an Send-As or Forwarding permissions assigned on the Mail-Enabled Public Folder.
  • When creating the Shared Mailbox, give it an SMTP address that does not match any of the SMTP addresses in the Mail-Enabled Public Folder. This way you do not run into a SMTP conflict.
  • When moving addresses, it is better to modify the Mail-Enabled Public Folder SMTP addresses rather than mail disabling it. This way, if you run into a problem, you can easily revert back to the original Mail-Enabled Public Folder by simply removing the prepending text to the one or more SMTP addresses.

Plan for the Public Folder to Shared Mailbox Transition

At some point during the migration you will want to disable access to the Public Folders on the source that you migrated. This will force the users to reference the new Shared Mailbox destination moving forward. To do this, you can simply remove permissions on the one or more Public Folders that were migrated. If you run into a problem you can always restore permissions to the source Public Folders, which is why this is a better option than deleting the source Public Folders right away. Once you confirm that the users are able to access the new Shared Mailbox then you can proceed with removing the migrated Public Folders.


Taking the time to understand which business processes are currently applied to your Public Folders will help greatly in your migration efforts. With the proper planning beforehand, you can be successful migrating away from Public Folders to Shared Mailboxes


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