We are making it easier to move your existing public folders to Exchange Online modern Public Folders. We are introducing 2 improvements to make this migration easier – first we have increased the size of public folder mailboxes from 25 GB to 50 GB – increasing the standard storage size for modern Public Folders in Exchange Online to approximately 2.5 TB. We are also making it possible for Exchange Server 2003 customers to utilize an Exchange Server 2010 Hybrid to enable Public Folder migration to Exchange Online. These updates help remove challenges in migrating your existing public folders to Exchange Online. Let’s have a look at the benefits and impacts of these changes.

Increasing Exchange Online Public Folder Mailbox Size

The larger public folder mailbox size provides you the ability to store a larger amount of data in modern public folders. Each tenant provides 50 public folder mailboxes now with a 50 GB quota – thus Exchange Online provides approximately 2.5 terabytes of public folder data in the cloud. This removes challenges we have seen in some migrations where the total volume of data exceeded the prior 1.25 TB limit.

The increase in mailbox size does not increase the size of a single public folder. We continue to recommend that you limit the size of any single public folder to 15 GB prior to migration. This recommendation is two-fold – it provides the best service experience and it avoids situations where public folder splits are required at migration. This limit is truly for a single folder – it does not include child folders for the calculation of a folder size. You should review the size of your current public folders prior to migration and reduce the size by deleting or separating large single public folders to ensure the best public folder experience in the service.

As the service is enhanced with changes, you should remember you still have control as the service administrator. You retain the control to reduce the public folder mailbox quota – however you are prevented from configuring quotas values larger than 50 GB in the service.

In the Exchange Online public folder mailbox quota is managed for you – the system monitors and automatically triggers public folder splits to best utilize use public folder mailboxes in the service. Splits are managed by the service and are transparent to users and service administrators. The end result of a split process is to keep all public folder mailboxes within the service limits.

Migrating Exchange Server 2003 Public Folders

This change helps customers migrating from Exchange Server 2003 to Exchange Online – as we have previously noted you can’t directly migrate Exchange Server 2003 Public Folders to Exchange Online. Now you can obtain and use the Exchange Server 2010 SP3 Hybrid to host your existing public folders on the hybrid server for purposes of migration. This migration to Exchange Online requires you move your existing public folder replicas to the hybrid server so there are no replicas left on Exchange 2003. We make multiple hybrid server keys available to you to retain public folder high availability on-premises during your co-existence phase. The scope of use for the Exchange 2010 Hybrid server in this case is limited to temporarily hosting on-premises Exchange Public Folders, solely for the purpose of migrating such Public Folders to Exchange Online. You can request a hybrid server key from Office 365 support.

What’s Next for Public Folders

We are working to provide additional control in the provisioning of public folders. In the future we will extend the New-PublicFolder command to specify an existing or new public folder mailbox, thus enabling admins to control the destination of public folders and avoid situations where a split process is required soon after migration. This is extremely useful when performing PST based public folder migrations.

Learn More About Public Folders

For complete details about Public Folder migration read the TechNet Article for Public Folders in Exchange Online.

Special thanks to the entire Public Folder Feature Crew and Brian Day for contributing to and validating this data.

Brian Shiers