Office 365 Public Folder mailbox limits and what they mean for migration

Microsoft’s Office 365 Public Folder mailboxes are the new home for legacy Exchange public folders.

But not so fast!

Sizing limits of Office 365 Public Folders are important to consider before doing a migration. Your current public folder hierarchy may not fit so nicely into the “modern” Office 365 Public Folder Mailbox architecture that Microsoft has designed.

Public folder mailbox sizing limits in Office 365 are important considerations before doing a migration. Here are some of the limits that might pose a problem for you:

  • Max. number of public folders: 250,000
  • Max. number of sub folders in a public folder: 10,000
  • Max. number of items per folder: 1,000,000
  • Max. number of public folder mailboxes: 1,000

Microsoft’s documentation currently states that mailboxes have a limit of 50 GB, but in our Office 365 tenants over the past few weeks, we are noticing mailbox size limits of 100 GB (Documentation often lags behind reality).

With 1,000 Office 365 public folder mailboxes at 100 GB, you have a max. data volume of 100 TB for your public folders. That’s fantastic, especially considering that you do not pay for them! Yes, that is right. Currently, any size Office 365 tenant can spin up the maximum public folder mailbox allowance.

For smaller companies with only a few hundred gigabytes of public folders, the limits on public folders are not an issue. However, for larger organizations, these limits present a real challenge. Not only is public folder migration to Office 365 a challenging process to begin with, but now you need to worry about transposing your legacy public folder hierarchy with these restrictions in mind. To even begin down that path, you need to have analytics on your current public folders.

We hope that Microsoft will significantly raise the restrictions on folder, subfolder, and item counts for public folder mailboxes.


Well, for starters, some of the limits are just too low. For example, in a recent conversation with a smaller construction company, their small 50 GB public folders contained nearly 8,000 subfolders under one folder. Right there, with a tiny migration scenario, the subfolder limit almost became a factor.

If smaller organizations come close to the limits, you can bet the bigger companies will hit them every time.

Any sizable public folder hierarchy (i.e. 25 TB and up) is likely going to exceed the 100,000 public folder limit by far. This restriction on the public folder count means that larger enterprises have a bigger project on their hands to dig into their current folders to decide what needs migrating and what can be archived or deleted. For instance, we recently spoke with a firm that has over 2,000,000 folders in their current hierarchy, but less than 100 TB. They obviously have a big cleanup job before migrating, and their migration will likely entail the data having to be spread across Groups, Shared Mailboxes, Office 365 Public Folder Mailboxes, and a cloud archive.

Here is what you need to be smart about sizing limits when you migrate public folders to Office 365:

  • Analytics – You need insight that can give you the full picture of your legacy public folders. Maybe you have iTunes Libraries sitting in public folders that aren’t worth migrating.
  • Intelligence — The insights you gain from analytics need to be actionable in the migration process. Not by you. The software should have fancy algorithms that use the insights to simplify the migration process.
  • Sizing Automation – You should not have to manually create Office 365 public folder mailboxes for the migration, and worry about what migrates where.
  • Sizing Optimization – To get the most out of your public folder mailboxes, the migration should allow you to optimize the structure of your content.

Microsoft has been trying to phase out public folders since SharePoint came along. The reality is that they are mission critical for many businesses, and because of client demand, Microsoft has been forced to keep them going.

Modern Office 365 public folders are a great solution, especially if you are migrating legacy public folders into Office 365 and you want a seamless transition for your users. Unfortunately, Microsoft is being somewhat stingy with their sizing restrictions on public folders today. We have every reason to expect this will change since limits in the cloud are usually being nudged higher based on demand. Several years ago, Microsoft announced Exchange Online Mailboxes at 50 GB a piece, up from 25 GB.

If you have concerns about your public folder migration bumping into the Office 365 sizing limits, you can contact us to speak with a technical specialist for some ideas on how to tackle it.

Become An Exchangesavvy Partner

Enhance Your Service Offerings With Proven Solutions.