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Must-knows for migrating Public Folders to Exchange 2013

Be aware of the limitations that come with moving Modern Public Folders in an Exchange 2013 migration.

Public Folders have long been the neglected, unloved Exchange Server feature many IT professionals wish would go...

away. They've stayed around for longer than many have expected because they work well from an end user's perspective.

If an organization is moving to Exchange Server 2013, some back-end issues, such as Public Folder replication, become history. Modern Public Folders allow Public Folders to be stored within Mailbox Databases and inside Public Folder mailboxes.

Migrating Public Folders to Exchange 2013 is a tough task. Before cracking open the champagne, admins need to be aware of the limitations of migrating Public Folders that could make them wait a while longer before a migration, reorganize their Public Folder infrastructure before attempting to migrate it -- or leave Public Folders behind for good. Take each of these issues into account before considering a migration.

Modern Public Folder maximums and limitations

Uncontrolled use of Public Folders within larger organizations can result in an unexpected surprise when planning an Exchange Server 2013 migration. It's reasonable to expect that limits will go up, not down. But due to the new architecture of Modern Public Folders, limits have been introduced where none were before.

Total number of Public Folders. The overall number of folders an organization can migrate was low with the original introduction of Exchange 2013. As of April 2015, that number stands at 1 million, which is enough for most organizations. Not only do admins need to take into account their current Public Folder count, but if they plan to keep using Modern Public Folders after migration, they need to plan accordingly for the future.

Although Microsoft hasn't released any plans for the limits they're aiming for, it's fair to say this is one area where improvement has been seen on a regular basis. The original limit of 10,000 increased ten-fold to 100,000 and then to 250,000 before reaching the current 1 million mark -- all within a year. Before admins embark on a pruning exercise, it may be worth speaking to their Microsoft account manager and making sure all requirements are flagged up.

Maximum Public Folder depth. For organizations with a deep, structured Public Folder infrastructure, the depth of sub-folders could catch them offguard. Although I've worked with organizations with more than 300,000 Public Folders that don't come near the depth limit, bear in mind that the maximum levels of Public Folders nested within each "branch" of the Public Folder tree is limited.

Maximum number of sub-folders beneath each node. Within each parent folder, organizations can only have a maximum of 1000 subfolders directly beneath. In reality, it's nearly impossible for end users to manage opening a folder that contains so many folders within it. Note that this limit doesn't include subfolders further beneath -- this just applies to what's directly attached to each node.

Public Folder size limits. Organizations using mail-enabled Public Folders may encounter another issue. If admins use mail-enabled Public Folders as "shared" mailboxes, the limit of 10 GB per folder may come back to bite them. Practically speaking, a folder of this size may be hard, if not impossible, to use with Exchange 2007 or Exchange 2010, but a long-forgotten notifications inbox might catch admins by surprise.

Limitations on future Office 365 migration options

Although Exchange Online is based upon Exchange 2013, the same limits don't apply to both services. The number of Public Folder mailboxes is halved -- limited to 50 Public Folder mailboxes of up to 50 GB each. The overall limit for Public Folder content size is limited to 2.5 TB, approximately a quarter of the total volume achievable with Exchange 2013 and half the number of supported Public Folder mailboxes.

The number of Public Folders supported is not in line with on-premises Exchange. Only 100,000 Public Folders are supported within Office 365, and there are no publically announced plans to increase this limit.

However, the most serious issue is that it isn't possible to move Public Folders from Exchange 2013 to Exchange Online using Microsoft's native tools. Admins have two options if they plan on migrating to Exchange Online at some point in the future and they must retain access to Public Folders. Admins can either keep their Public Folder infrastructure on a legacy version of Exchange until they're ready to migrate, or they can plan for a hybrid Exchange infrastructure with Public Folders remaining on-premises.

The one positive aspect of this is that Public Folder access is one area where end users both on-premises and in Office 365 remain able to access the on-premises Public Folder infrastructure with little to no impact. Unlike shared mailboxes and other delegated scenarios where everyone using those resources must migrate together, Public Folders end users can be moved to and from Exchange Online with no impact to Public Folder access.

Folder access issues for users in multiple geographies

One often overlooked factor with planning for Public Folder access is the changes and impact on end users who access Public Folders from multiple geographic locations. Legacy Public Folders uses a multi-master replication model, allowing IT administrators to replicate content to the servers closest to end users, thus avoiding latency issues when accessing content.

Compliance, in-place hold and auditing

One big caveat with migrating Public Folders to Exchange 2013 could block migrations where data within Public Folders is subject to compliance and auditing requirements.

In particular, in-place hold and legal hold functionality can't be used with Modern Public Folders. This means if data such as email must be stored unaltered for a certain period of time, it must either be moved to a shared Mailbox or other platform or journaling must be used along with a third-party system. Built-in functionality for mailbox access audit logging, a functionality used to track who accesses mailbox data, isn't available.

About the author:
Steve Goodman is an Exchange MVP and works as a technical architect for one of the U.K.'s leading Microsoft Gold partners. Goodman has worked extensively with Microsoft Exchange since version 5.5 and with Office 365 since its origins in Exchange Labs and Live@EDU.

Next Steps

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This was last published in April 2015

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How about sharing some Best Practice when migrating Exchange public folders to Shared Mailboxes in O365? How would you go about doing it?
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