Migrating More than 20GB of Public Folder Data to Microsoft 365

Microsoft 365 has several hard limits for Public Folders.
Though the size limit of Public Folder mailboxes can go up to 100GB, when a Primary Hierarchy Public Folder mailbox gets close to the 20GB limit, Microsoft 365 will detect this and can create another Public Folder mailbox, and then run a rebalancing operation in order to keep the aggregate size of all folders in a given mailbox below its limit. This auto-split operation can take up to two weeks to complete and can block access to the affected public folders.
When migrating data, we add items to Public Folders much faster than Microsoft 365 is expecting them.
In order to avoid triggering the auto-split operation, an operation has to be performed before a migration begins that will properly pre-allocate Public Folders mailboxes to cater for all the migrating data.

If running a Public Folder migration, and there is more than 20GB of data that you are migrating, contact BitTitan Support so that we can generate the PowerShell scripts necessary to properly provision the Public Folder mailboxes, as well as the script necessary to allocate the Public Folders to the correct Public Folder mailboxes to keep the aggregate size below the 20GB limit. Be sure to select "Support" to ensure your request is routed to the correct team.

​If this is not done, once the 20GB limit in the primary Public Folder mailbox is reached, it will be necessary to wait for the rebalancing operation (by Microsoft) to complete before continuing the migration, which can take up to two (2) weeks.

Here is the correct steps to follow when performing a Public Folder migration, if the total of the Public Folders is greater than 20GB:

  1. Create a Public Folder migration project.
  2. Select the Source and enter admin credentials.
  3. Select the Destination and enter admin credentials.
  4. Specify the Source path for the project.
  5. Submit a ticket to Support by following the directions in How do I get support for your products?. In the ticket description notes, add the name of the Public Folder migration project, and specify that the total of the Public Folders is greater than 20GB.
  6. Upon receipt, Support will send you a confirmation.
  7. Support will then run scripts that will allocate Public Folders to the correct Public Folder mailboxes in order to allow the mailboxes to stay below the 20GB limit. This can take several days to complete. For example, if the total Public Folder size is greater than 100GB, it will likely take more than two (2) days. This script will query the Source environment to calculate the size of each item, the number of items in each folder, etc. 
  8. Once the Public Folder mapping script has completed, Support will provide two (2) PowerShell scripts. These will be supplied via the Support ticket system. These must be run by you against the Microsoft 365 environment. Instructions will be included with the scripts. These scripts will complete within a matter of minutes.
    Note: If running Public Folders in hybrid mode, refer to Why do I get an error when I try to run the Public Folders mapping script? prior to running these scripts; the article will explain the required steps to follow in order to be able to run the scripts successfully.
  9. Once complete, the Destination environment will be ready to begin the Public Folder migration.
  10. Now log in to MigrationWiz, select the Public Folder project that was previously set up. Then follow the steps in the relevant Public Folder Migration Guide; for example, when the Source is On-Premises Exchange, refer to Public Folder Migration Guide From On-Premises Exchange 2007+ to Microsoft 365.

Auto Split

Auto Split is triggered at the 20% threshold for the Primary PF Mailbox, and at 70% for Secondary PF Mailboxes. This is performed earlier against the Primary PF Mailbox to ensure that resources required to master the PF hierarchy are not strained.
PF mailbox size 100 GB - only putting data in the primary PF mailbox – auto-split at 20% - 20 GB
PF mailbox size 100 GB - putting data in the secondary PF mailboxes – auto-split at 70% - 70 GB

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