We are happy to reveal that we’re increasing the number of public folder mailboxes in Exchange from 100 to 1,000. More Public Folder mailboxes means more storage space capacity for public folders. This increase will facilitate the migration of very large Public Folders from on-premises Exchange Server to Exchange Online. This increase has already begun rolling out to Exchange Online tenants, and should be completely deployed by mid-February.
For customers who maintain their own on-premises Exchange Server 2016 infrastructure, the increase is expected to become available via Exchange Server 2016 CU2, in the second quarter of CY2016 (Edit July 2016: this has now been released). On-premises customers will then have more flexibility regarding placement of data in remote offices where necessary to give primary users a lower latency experience. It is possible to do this with the existing 100 Public Folder mailbox limit; however, some customers needed to make trade-offs between how many Public Folder mailboxes they could place in regional Exchange locations, and how many they needed for data storage in the main location. Growing the Public Folder mailbox limit to 1,000 will help those larger customers offer the lowest latency experience to their users, regardless of where they’re located.