The term “Modern Public Folders” is Microsoft’s wording to express their new architecture for Public Folders in Exchange Server and Exchange Online in which public folder data is hosted in special mailbox containers. Modern Public Folders first appeared in Exchange 2013. “Legacy Public Folders” is the term used to refer to Exchange 2010 and earlier implementations that used a monolithic database as the container for public folder data.
Note that for Office 365, the public folder mailboxes do not incur any additional licensing cost. For all versions of Exchange, public folder mailboxes are not considered “shared mailboxes”, and do not appear in the list of user or shared mailboxes. The end-user experience for modern public folders is the same as with legacy public folders and users will not notice any visible differences. Exchange 2016, 2019, and Office 365 support up to 1000 public folder mailboxes for which to distribute content. Office 365 only supports up to 10TB of data while on-premises environments support up to 100TB.
The following table is sourced from this Microsoft article: Public Folder Limits
| Item | Limits | Notes |
|---|---|---|
| Total number of public folder mailboxes | 1,000 | 1,000 is the limit for Exchange Server 2016 CU2 or later. Although you can create more than 1,000 public folder mailboxes, it is not officially supported. See Create a public folder mailbox. |
| Total public folders in hierarchy | 1,000,000 | Although you can create more than 1,000,000 public folders, it is not officially supported. For any deployment of 100,000 or more public folders, we recommend reading Considerations when deploying public folders. |
| Sub-folders under the parent folder | 10,000 | Although you can create more than 1,000 sub-folders under a parent folder, it is not recommended. The limit can be enforced with the FolderHierarchyChildrenCountReceiveQuota parameter on the Set-Mailbox cmdlet. |
| Folder depth | 300 | The folder depth is the number levels of nested folders that can exist in one branch of a public folder tree. The limit can be enforced with the FolderHierarchyDepthReceiveQuota parameter on the Set-Mailbox cmdlet. |
| Maximum messages per public folder | 1 million | The limit can be enforced with the MailboxMessagesPerFolderCountRecieveQuota parameter on the Set-Mailbox cmdlet. |
| Maximum individual public folder size | 10 GB | This limit doesn’t include subfolders beneath a single folder. See Configure storage quotas for a mailbox. |
| Public folder mailbox size | 100 GB | Although public folder mailbox size can exceed 100 GB, it is not officially supported. See Configure storage quotas for a mailbox. |
| Number of user logons per public folder mailbox | 2,000 concurrent user logons | We recommend that you configure your hierarchy so that you have no more than 2,000 users per public folder mailbox. For example, if you have 20,000 users, you should have 10 public folder mailboxes. |
| Moved item retention | 14 days recommended | Use the DefaultPublicFolderMovedItemRetention parameter on the Set-OrganizationConfig cmdlet. |
| Age limit | We recommend that you set this as the same default that you use for regular mailboxes. | These settings can be set at the following levels: Organizational level: Use the DefaultPublicFolderAgeLimit parameter on the Set-OrganizationConfig cmdlet. Folder level: Use the AgeLimit parameter on the Set-PublicFolder cmdlet. |
| Deleted item retention | We recommend that you set this as the same default that you use for regular mailboxes. | These settings can be set at the following levels: Organizational level: Use the DefaultPublicFolderMovedItemRetention parameter on the Set-OrganizationConfig cmdlet. Mailbox level: Use the RetainDeletedItemsFor on the Set-Mailbox cmdlet. Folder level: Use the RetainDeleteItemsFor parameter on the Set-PublicFolder cmdlet. |
| Maximum number of public folders that can be migrated from Exchange 2010 to Exchange 2016 | 500,000 | This is the maximum number of public folders you can move to Exchange from Exchange 2010 in a single migration. Although you can attempt to migrate more than 500,000 folders, it is not officially supported. For details on migrating public folders, see Use batch migration to migrate public folders from Exchange 2010 to Exchange 2016. |
The Priasoft PF Sync solution supports the sync and migration of public folder between either type of implementation. It can handle legacy-to-legacy, modern-to-modern, legacy-to-modern, and modern-to-legacy patterns. The solution can also handle any mixture of source or target on-premises and Office365 environments. Given that most public folder migration scenarios are a move to modern public folders, this article will focus on those scenarios.
