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Modern Public Folders in Exchange Online

Beginning with Exchange 2013, Public Folders underwent a change in name. From 2013 onwards Public Folders are referred to as Modern Public Folders and the previous versions are known as Legacy Public Folders. However, it was much more than a name change.

The Public Folder databases have been removed, and have been replaced with the new Public Folder Mailboxes. These new public folder mailboxes contain the hierarchy information as well as the content for your public folders. Modern Public Folders offer the same user functionality as Legacy Public Folders, but also align with Microsoft’s efforts to improve high availability using Database Availability Groups (DAGs).

How do Modern Public Folders work in Exchange Online?

Before you can create public folders in Exchange Online, you must first create at least one public folder mailbox. The first public folder mailbox that you create is known as the primary, and it contains the only writable copy of the public folder hierarchy (primary hierarchy). As the content of the public folders increases beyond the capacity of the primary public folder mailbox, Microsoft automatically creates a new public folder mailbox called a secondary to accommodate your public folders. Secondary public folder mailboxes contain a read-only copy of the primary hierarchy (called the secondary hierarchy).

The public folder hierarchy contains the folders’ properties and organizational information, including tree structure.

Public folder content can include email messages, posts, documents, and eForms. The content is stored in the public folder mailbox but isn’t replicated to other public folder mailboxes. All users access the same public folder mailbox for the same set of content.

Modern Public Folders in Exchange Online
Click image for larger version

Can I migrate my Legacy Public Folders to Exchange Online?

Yes, public folders are supported in Exchange Online, and legacy public folders can be migrated to Exchange Online. However modern public folders and legacy public folders cannot exist in your Exchange organization simultaneously. This effectively means that there is no coexistence. Migrating public folders to Exchange Online is currently a one-time cutover process.

If you’re running Exchange 2003 in your organization, you cannot migrate public folders directly to Exchange Online. You will need to move all public folder databases and replicas to Exchange 2010 SP3 RU8 or later, or Exchange 2007 SP3 RU10 or later first.

How do you do this?

There are currently two methods available to migrate your legacy public folders to Exchange Online:

Batch Public Folder Migration (recommended)

This approach allows you to move existing mailboxes instead of creating new user mailboxes and importing user information. Permissions automatically migrate at the folder level with the data, you don’t have to perform this step separately.

Outlook PST export

This approach is not recommended if you use complex permissions or have many folders to migrate.

Also do not use this migration method if the on-premises public folder hierarchy is greater than 30GB, as public folder mailbox growth is managed using an auto-split feature that splits the public folder mailbox when it exceeds its size. As auto-split can’t handle the sudden growth of public folder mailboxes when you use PST export, you may run into an issue where the primary mailbox becomes full and you have to wait for up to two weeks for the auto-split to complete.

Public folder permissions will also be lost during this migration. It is recommended to capture the current permissions before migration and manually add them back once the migration is completed.

Also, be aware that any item and folder changes made to the source public folders during the PST export migration will be lost.

What are the Modern Public Folder limits for Exchange Online?

The limits below currently apply to all Office 365 Business and Enterprise plans

Exchange Online


Number of public folder mailboxes


Public folder mailbox size


Total size of all the public folder mailboxes combined



It you wish to discuss moving your legacy public folders to Exchange Online, please feel free to contact Intergen.

References for even more information:

Posted by: Leighton Childs, Infrastructure Consultant, Cloud Design and Integration | 19 September 2016

Tags: MIcrosoft Exchange

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