Public Folder Migration: What to Expect?

Public Folder migration can be hard. It requires a lot of planning, patience, and resolve to get through it. Knowing what you are up against will be the key to avoiding end user impacting issues. If you are choosing to migrate public folders to Office 365, then it’s even more crucial.

Public Folder Migration Process Overview

There are four phases of a Public Folder Migration and each build upon the previous one. Analysis, Provision, Migrate, and Finalize. Here’s what you can expect during each phase:


Knowing what your Public Folder layout consists of is very important. Here is just some of the factors you will need to be aware of during this phase:

  • Total amount of content (in MB, GB, or TB).
  • Total number of folders.
  • Any items over 150 MB in size.
  • Any single folder greater than 50 GB.
  • Total number of mail-enabled Public Folders.
  • Whether or not special templates are used.

The version of Exchange you are migrating from also plays a role in this phase.

Migrate Exchange 2010 Public Folders to Office 365

In this scenario, you will be moving from Legacy to Modern Public Folders. Legacy Public Folders have their own database within Exchange. With the move to Office 365, Modern Public Folders are used, which means that special mailboxes hold the Public Folder structure and content. Planning your migration at this phase involves mapping folders to public folder mailboxes. If you have a large Public Folder structure, this can be a difficult and long process. Not only do you need to consider Office 365 limits, but you need to account for extra data being added. Users will want to continue to use Public Folders while you are migrating, so it must factor into your mapping.

Migrate Exchange 2013 Public Folders to Office 365

This scenario is not much different than the Exchange 2010 one. The concept of Modern Public Folders was introduced in Exchange 2013. It’s easy to assume that this can uplift to Office 365, but this is not the case. The limits imposed by Office 365 prevent this from happening. You will still have a good amount of folder-to-mailbox mapping to do.

House Cleaning

This is a good time to consider archiving old or unused content. This will save you time figuring out where to map folders that have no value to your business.


Once you have an idea of what you will be migrating, the next step is to provision the target. Most likely, that target is Office 365. Unless you have a very simple Public Folder structure, you will not be able to simply translate that over to Office 365. Even if you are already using modern Public Folders on premise, Office 365 has different limits. You will need to factor those limits into play when deciding how to distribute your Public Folder structure. This phase can take a long time depending on how large your Public Folder structure is.


Depending on how much content you have, the actual public folder migration process can take days or weeks. Delays can occur that extend your timeline, and Auto-Splits are the biggest threat to this. Auto-Splits are a nice feature in Office 365 in that it’s a hands-off approach to adding more Public Folder mailbox capacity. However, this was not designed for use during a migration, where content is being added quickly. If you trip an Auto-Split, your Public Folder structure can be locked down for up to two weeks!

How and where Public Folders are accessed must be considered. For example, if Exchange 2013 is on premise, users still on premise will be able to access Public Folders hosted on Office 365. If Exchange 2010 is on premise, then users still on Exchange 2010 will not be able to view Public Folders hosted in Office 365. This is important to know as it will impact the timing of your migration. It’s often why you see Public Folders being the last item to migrate over to Office 365.


This phase involves officially cutting over and using your Public Folders in Office 365. The speed of this process is dependent on the total number of folders that you have. With larger amounts of folders, it can take days for this process to complete. A requirement through the Microsoft API is that the source Public Folders cannot be accessible during this time.

Where your organization and users point to for Public Folders is also changed during this phase. This change is noticed with Outlook as the Public Folder tree will disappear and then reappear.

Mail-Enabled Public Folders are permanently changed during this phase. Only some of the properties of these folders are kept. Once complete, these folders will no longer exist on premise.


Public folder migration is not easy. There’s a lot of planning involved and lots of technical details to understand. You are responsible for translating the requirements in Office 365 to fit with your Public Folder structure. The version of Exchange that you are migrating from plays a role in how you plan and execute. There are steps during the Finalize phase that are not reversible.

These are difficult migrations, but they don’t have to be. With ExchangeSavvy’s Public Folder Migrator, you can have all of these phases taken care of for you automatically. Regardless of how large and vast your Public Folder structure is, we can help you tackle it with ease. Please contact us today to discuss your Public Folder migration needs!

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