Public Folders has long been a feature in Microsoft Exchange as a way to help facilitate collaboration amongst users well before collaboration tools hit the market. Public Folders offer an ability to create folders where files and emails can be placed into them and access granted to groups of users for common access. You can also mail-enable the folders, allowing them to send and receive mail using a simple alias. All of this is available through using the Microsoft Outlook Client, which makes it that much more appealing as a one stop application for these needs without having to roll out a separate client. Because Public Folders have been around for a long time, companies have embedded organizational processes into them as the very fabric of how they operate day to day. They use them as a central repository for team data, or for project managed activities as examples. Over time, the number of folders contained could number in the tens of thousands and data in the Terabytes spread amongst multiple Exchange servers. Maintaining Public Folders over time has also shown to be problematic, as versions of Exchange up through 2010 had them stored in dedicated databases outside of where mailbox data was stored, so you did not have the ability or flexibility of using DAG’s (Database Availability Groups) to provide high availability and disaster recovery. You could get around this by custom scripting, but not having it by default imposed additional administration burden. Public Folders were since included as part of the DAG as of the release of Exchange 2013. With Exchange 2016, you can support up to 1 million Public Folders! However, again, because it’s part of the DAG and uses Mailboxes to store the Public Folder hierarchy, you need to be aware of how much space is being consumed per public folder mailbox as this can impact accessibility to your end users. Now that Public Folders are officially supported in Office 365 Exchange Online, you can move them and no longer be burdened with all of the management headaches you have had before…well, for the most part. The significant gains in administrator productivity are evident, where the pressures of maintaining high availability and disaster recovery for your Public Folder hierarchy are in the hands of Microsoft. Exchange Online is even able to automate mailbox splits, so when your Public Folder mailbox approaches the 100GB quota limit, it will already partition it off to another mailbox. As I mentioned previously, with most organizations business processes tightly wrapped around Public Folders, keeping it accessible and protected from disaster are paramount. However, moving Public Folders to Office 365 Exchange Online does not completely absolve the Administrator from keeping tabs on them. Some conditions must be strictly followed to keep public folders accessible, so knowing them beforehand is critical. These conditions are not only necessary for maintaining a working Public Folder environment but also play a crucial role in migration planning to Exchange Online, as failure to account for them can severely set back your efforts. So, when planning for a migration of Public Folders to Office 365 Exchange Online, it would seem that the migration should be easy as long as you understand the conditions and have planned for them, but it is anything but easy. If you are migrating from Exchange 2010 for example, you will be faced with a cumbersome script-based public folder migration process that can be daunting for even the most seasoned administrator. Even if you can manage the scripting operations, you still need to map your Public Folder hierarchy to be compatible with Office 365, which depending on the size of your folders can be an extremely time-intensive task. In addition to this, you will also need to know if you could potentially hit limits on the number of sub-folders as Exchange Online has limits on this whereas there are none when it comes to on-premises Exchange. Once you have your Public Folder hierarchy mapped to be compatible, you then need to prepare for the actual migration. Depending on the version of Exchange you are migrating from, speeds could be a lot slower than anticipated. For example, in Exchange 2010, it is not uncommon to see transfer rates between 5 and 10 GB/Hour. If you have Terabytes of data, be prepared for the migration to take days, if not weeks. The other aspect of the migration is whether or not you want your users to continue to create new Public Folders and add content to them during the migration. If you have a lot of data and project the migration to take days or weeks, it may not be acceptable to the business to have them locked down for that long. If that is the case, then you need to prepare to run multiple migration passes as to account for the delta between migration runs, and the comparison between the two is a manual process. While keeping your Public Folders may be necessary, moving them is not impossible, it just requires much planning and thought into how the process should work for your circumstances. The following is an excellent presentation given by a Microsoft MVP Peter Schmidt in regards to how to migrate your Public Folders to Office 365 Exchange Online.