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This article is part of our Essential Guide: Plan your Exchange migration to Office 365 with confidence

Migrating public folders requires extensive planning

Do your homework to find the right option for public folders when making a move to Exchange Online. Here are guidelines to help you navigate the requirements and limitations.

When a company wishes to move to Exchange Online, it must decide if migrating public folders to Exchange Online public folders, converting them to Office 365 Groups or keeping the public folders on premises is the right choice.

Public folders, a feature in Exchange Server since its 1996 debut, let users share content stored in hierarchical folders. Organizations that plan a move to Exchange Online must decide how to handle on-premises public folders. As Office 365 has matured, so have the options to address an organization's public folder coexistence and migration requirements.

Public folder access is important

An Exchange hybrid deployment enables an organization to migrate mailboxes from Exchange on premises to Exchange Online at its own pace. It also provides consistent access to public folder content during and after the mailbox migration.

Microsoft provides different ways to address public folder access requirements depending on the location of user mailboxes:

  • access public folders on premises during and after a migration;
  • migrate public folders to Exchange Online public folder mailboxes; and
  • migrate public folders to Office 365 Groups.

Microsoft documentation often refers to public folders stored in Exchange 2007 or Exchange 2010 as legacy public folders. Public folders stored in Exchange 2013 and above are referred to as modern public folders. Microsoft does not support a hybrid configuration with public folders stored in Exchange 2003.

A mailbox migration requires some adjustments

An organization must configure access to on-premises public folders if some or all of its mailboxes have migrated to Exchange Online. After the mailbox migration, an organization can choose to migrate its public folders or keep them on premises.

Permitting Exchange Online mailboxes to access on-premises public folders requires many technical and operational considerations. For example, Outlook Anywhere must be enabled and functional within the on-premises Exchange environment.

Permitting Exchange Online mailboxes to access on-premises public folders requires many technical and operational considerations.

Organizations using Exchange 2010 might need to adjust their architecture because the Client Access Server role needs to be on all the mailbox servers that contain a public folder database. IT must create public folder proxy mailboxes in the on-premises system to permit Exchange Online user access.

Microsoft scripts can help with other configuration requirements, such as the synchronization of on-premises, mail-enabled public folder objects into Azure Active Directory. This ensures the public folders appear in the Global Address List of users with mailboxes that have migrated to Exchange Online.

Organizations should review Microsoft's guidance for legacy -- Exchange 2007 and Exchange 2010 -- and modern -- Exchange 2013 and Exchange 2016 -- scenarios before work starts on a hybrid configuration for public folder access.

Migrating public folders to Exchange Online

A public folder batch migration to Exchange Online is a complex process with prerequisites and technical considerations.

If an organization wants to migrate fully to Exchange Online, yet needs to retain its public folder content, it can migrate the legacy or modern public folders to Exchange Online public folder mailboxes. Using this method, the public folders will look the same in Outlook. Called a batch migration, this method uses PowerShell batch migration commands to complete the task.

Larger public folder hierarchies are now supported

Public folders can only reside in a single place, either in Exchange on premises or Exchange Online. Microsoft had capped the number of public folders in Exchange Online to 100,000, but announced two changes in September 2017 to support organizations with larger public folder hierarchies.

  • Organizations with Office 365 plans can have an Exchange Online public folder hierarchy with up to 250,000 public folders. This enhancement does not apply to stand-alone plans, which remain limited to 100,000 public folders.
  • Organizations running Exchange 2010 can migrate up to 250,000 public folders to Exchange Online.

Organizations on Exchange 2013 or Exchange 2016 can only migrate up to 100,000 public folders to Exchange Online. After the migration finishes, the organization can expand the Exchange Online public folder hierarchy up to 250,000 public folders. Microsoft plans to remove this limitation for Exchange 2013 and Exchange 2016, so organizations with these versions and a large public folder hierarchy should watch for developments.

Migrating public folders to Office 365 Groups

Public folder mailboxes are no longer the only target location within Exchange Online for Exchange 2010 or Exchange 2013 on-premises public folders. Companies can also use a batch migration process to migrate public folders in Exchange 2010 or Exchange 2013 to Office 365 Groups. An organization already using public folders in Exchange Online can migrate them to Office 365 Groups.

Office 365 Groups have several benefits over public folders. They provide a shared mailbox with historical conversations, a shared calendar with group event subscription, a SharePoint team site for document-based collaboration with features such as document co-authoring, and a rich mobile experience via the Outlook mobile app. This functionality offers a compelling reason to switch when compared to the limited collaboration capabilities of public folders.

Prior to migrating public folders to Office 365 Groups, an organization will need to fully review Microsoft's guidance for legacy -- Exchange 2010 -- and modern -- Exchange 2013 -- scenarios.

This was last published in April 2018

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What plans do you have for the public folders in your Exchange setup?
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