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How To: Migrating Public Folders to Office 365


There are a number of elements you need to consider before attempting a Public folder migration to ensure a successful result.

Public folder Store Size

One of the more critical elements to consider before a migration is the size of the data you are going to be migrating. The MessageOps Migrator does item level migration meaning that each Item is copied to Office365 using the client API’s. This imposes a limit on the throughput of data that can be migrated per hour so one instance of the migrator will be able to move between 500Mb-1Gb of data per hour. This means a migration could take a long time to complete if you are trying to move a large amount of data eg 100 Gb would take at best 100 hours (or 4 days) to completely migrate. Large folder numbers can also be another element that can slow the migration down as each folder needs to be individual queried and enumerated. 

Folder Permissions and Unwanted Information Disclosure

As part of the folder hierarchy synchronization stage the existing folder permissions on Exchange source folder will attempt to be replicated in Office 365. Public folder permission can be broken down into two different types there are the default standard entries for

Default (which is basically every authenticated user)

And then there will be Entries for Users and Groups, for the transfer of permissions to work successfully the user and groups need to be able to be resolved on both sides of the Migration by default the primary SMTP will be used but this can be change to use the display name of the user as an alternative.  

As Public Folders are a shared resource between all users therefore there is a risk if you have a public folder with sensitive information, that after migration some users maybe be able to access data they should not have access to.   This is a critical Migration risk that needs to be managed. The Folder hierarchy and Data copy stages are separate steps in the Migration and you should ensure by verifying the permission on any folders that may contain sensitive information before you commence the data copy stage of the migration to ensure you don’t have any unwanted information disclosure.

Mail Enabled Folders

If you have mail enabled folders in your On Premise Exchange organization the Exchange Migrator does have an option that can be used to migrate these setting to Office 365. For the email address to be synced successfully the domain of the email address you want to migrate needs to be an accepted domain in Office 365 see  

Current Public Folder Condition and Replication Status

The final consideration that that needs to be taken into account is the health of your current public folder trees and folder replica’s. In more complex Exchange environments there can be more than one public folder database hosted on different Exchange servers within your organization. Multiple replicas and replication of public folders can be configured within these stores; therefore the replication status between the Public folder stores is critical. When performing the data migration the Exchange Migrator will logon to the OnPremise Exchange Store using MAPI, and migrate data from the Public folder instance that it is referred to. If this particular folder instance is not up to date or replication is not configured or working properly then not all the folder content will be able to be migrated. It’s strongly recommended therefore if you have a multi-server environment with multiple folder replicas that you performs some health reporting to ensure there are no problems in your On Premise Exchange environment before commencing the migration.

For more information on checking your Public Folder Content health see


Getting Started with Public Folder Migration

Before performing a Public Folder Migration you need to ensure your Office 365 tenant has been setup correctly for Public Folders. Before you perform the migration you need to ensure that Public Folders are enabled by creating a least on Public folder Mailbox see for details on how to do this.

On Office365 each Public Folder Mailbox can host up to 50 GB of data, so if you are moving more than 50 GB of data you need to create addition Public Folder Mailboxes to host the data. (The process of maintaining the data over multiple mailboxes is handled automatically by Exchange). For example if you have 100 GB of data to migrated you should create a least 3 Public Folder mailboxes to ensure adequate data space.

Stage 1 Folder Hierarchy

The first step in the process of doing a public folder Migration is to sync the Folder Hierarchy; this process can be broken down into three separate subtasks.



The FolderSync is the first step in the Folder hierarchy stage, during this process all the Folders in the OnPrem Exchange environment will be enumerated. Then a replica Folder Hierarchy will be created on the Office365 side. If you wish to do some test migrations with one folder you can do this using the Single folder checkbox see (“Doing Single Folder Migrations”)

To Start the FolderSync stage you need to first select the RootPublic folder where you want the migration to begin and also the destination in Office365 where you want the folder hierarchy to be created.

After you have selected both the Source and Destination folder you then have the option of either enabling or disabling the Folder Permissions stage of the folderSync Process (this is enabled by default) and the MailEnabled setting stage of the migration (this is disabled by default).

When you click the Start Hierarchy Start button the migrator will then try and synchronize the folder hierarchy. Depending on the number of folders you have and the complexity of the permission this task can take some time.  While it is synchronizing you will get a running verbose output in the in the Hierarchy sync log, to view the full output of the log in a text editor you can use the Open Log button.

Folder Permissions

If the Migrate Permission box is selected as part of the FolderSync task the folder permissions will also be attempted to be synchronized. Note the Account your using to do the Migration will always be assigned explicate owner rights during the folder creation process. These rights will be important to the Data synchronization steps for the data copy to complete successfully. You may need to run a separate process to remove rights for this account to folders that have sensitive information after the migration is complete. As Folder permission need to be processed individually and in some cases resolved this step can add significant time to folder synchronization stage.

Mail Enabled Folder Settings

If you select the checkbox to Migrate the Mail enabled folder settings once the application has finished folder synchronization step, the MailEnabled settings from the OnPremise Exchange environment will be attempted to be migrated to the destination environment. To perform this migration all the Public Folder proxy objects which contains all the email address assigned to the public folders in the source environment will first be enumerated. Then all the email addresses assigned within the destination will be enumerated. The source email address cannot be assigned to the destination folder if:

  1. The domain of public folder email address is not part of the accepted domains. (note some invalid proxy addresses maybe assigned to an existing public folders that will cause errors during this stage).
  2. The Address has already been assigned to another object in Office365

If the Primary email address assigned to the current source folder is not able to be synchronized for the above reason the application will log an error for this.

Folder Hierarchy Failures

Because Public folder hierarchy’s can be complex and hosted over multiple servers and stores the unavailability of one of these servers and store can cause the synchronization process to fail.

Doing Single Folder Migrations

You can perform Single Folder migrations with the Migrator which can be useful if you want to test the application or if you have critical folders which you need to prioritize.

 When doing Single folder Migrations the behaviour of the FolderSync and Data copy is different. When doing a Single Folder migration –

The Source Root Folder – Is the actual folder you want to Migrate

The Destination Parent Public Folder – This is the Parent folder where the Source Folder will be created as a Subfolder. If the folder already exists in Office 365 you need to select the Parent folder to copy data successfully.

Similar to the full hierarchy sync the Folder permissions and the Mail Enabled setting will also be migrated to the single folder.

Stage 2 Data Copy

Once you have successfully created the Destination folder hierarchy you can start the Data Copy stage by selecting the “Start Data Copy” button.

Data is copied one folder at time and every folder is passed two times to ensure that any errors that happen as part of the Item level copy, that the items involved will be at  least retried once before an error is returned. Like Mailbox Migrations Client Throttling affects the migration process, the destination admin account being used will affect the rate at which items can be imported. If you want to run multiple instances of the Migrator at one time it’s important to use separate Admin accounts as throttling is calculated on a per user bases and this aggregates across all connections the user creates/uses. 

Large Tree Behaviour Options

The Migrator offers the ability to perform an initial pass of the first X (1000 by default) items in a folder. If you have a very large folder tree with a large number of items this allows you to copy the most recent items in each folder first. When you select this option it means the Migrator will perform 3 pass of each folder. The first pass will copy the first (X) number of items, then the next pass will copy the remaining items in the folder and last pass will retry any failed items not copied in the first 2 passes.

Duplicate Handling Behaviour

The Migrator will flag each item it imports to ensure that no Item is imported more than once, however the Migrator doesn’t not perform a Synchronization of the folder items meaning that if changes are made OnPremise to Items after they are imported the modified Item will not be synchronized if you run an import on the folder again.

Troubleshooting Errors with the Data Copy

The data copy stage of the migration uses the same technology and method as the Mailbox Import stage so please see our Troubleshooting Exchange Migrator PST Import Errors to diagnose any errors.


To aid in the Migration and checking of the migrated folder the Migrator can run multiple reports

Source folder reports – The Source folder report button displays a report of the current source folder permission settings

Mail Enabled Folder reports - The Mail Enabled folder report shows the status of the Mail Enabled folders on the Source and Destination environments. 


Updated: April 16, 2016

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