Public folders in the new Office

Published Nov 08 2012 11:01 AM 31.7K Views

Modern public folders – ready for Office 365

We made significant architectural changes with modern public folders to deliver against the feedback we got from customers:

  • “Having a separate management and replication model is complicated and expensive.”
  • “We don’t get public folders in Office 365 so we cannot migrate to the service.”
  • “We want public folders, don’t take them away!”

Built on mailbox infrastructure

Building on known mailbox infrastructure greatly simplifies management and backup/restore for public folders. IT administrators no longer need to learn two different management approaches for mailboxes and public folders.

Furthermore storage cost can be significantly reduced by leveraging mailbox infrastructure. Future improvements in mailbox storage management will automatically accrue to public folders as well.

No more public folder replication required

High availability is ensured through the existing high availability service for mailboxes instead of managing public folder replication separately. Public folder replication is a thing of the past.

Scaling to Internet scale

The new public folders literally scale to Internet scale. As public folders grow, content simply spans out to a new mailbox. In Office 365 a new mailbox will be added automatically, on premises IT will fork out to a new mailbox.

Office 365

In Office 365 management and storage is handled by Microsoft. You can keep supporting your public folder business scenarios but outsource management and storage to Microsoft by moving public folder deployments to the cloud.

Terabytes of public folder data turns from a cost to a differentiating asset for the business.


Figure 1: Public folders in Office 365 Customer Preview

The architectural work and redesign we did for public folders in the new Office is really focused on the IT admin. Management and scalability is much improved over the previous architecture and public folders are now ready for the service while we kept the end user experience the same for this release.

Modern public folder architecture

Core concepts

Here are some of the core concepts of the modern public folder architecture. We will talk though their impact in the following paragraphs


  • Each public folder mailbox has a copy of the public folder hierarchy
  • There is only one writeable copy of the hierarchy at any given time
  • Clients connect to their home hierarchy


  • Public folder content is stored in a public folder mailbox
  • It is not replicated across multiple public folder mailboxes (although passive copies of the mailbox are supported through high availability services)
  • All clients access the same public folder mailbox for a given set of content

Let’s discuss next what those core concepts mean for some of the key IT scenarios.

Scaling out public folders

As public folders grow, the content will outgrow capacity of the existing mailbox(es). IT will have a script to split off a branch of the public folder tree and have its content be moved and stored in a new public folder mailbox (remember: that new mailbox will also host a full copy of the public folder hierarchy).

In Office 365 the content will be automatically branched out into a new mailbox as the threshold is reached.

Scale out flow

Existing public folder deployment with two mailboxes. Content for the folders ‘Contoso’ and ‘Sales’ is stored in mailbox 1, content for “Marketing’ and “Finance’ is stored in mailbox 2. The complete hierarchy is stored in each mailbox.


As content in mailbox 2 grows, the ‘Finance’ folder will be branched out to a new mailbox.


New content for the ‘Finance’ folder or new folders under that branch will be stored in mailbox 3.


It’s completely transparent for end users whether content is sto red in one mailbox or another.

Availability and redundancy

Modern public folders build on mailbox infrastructure and leverage the same mechanisms for availability and redundancy. Every public folder mailbox can have multiple redundant copies with automatic failover in the case of failures.

Failover flow

Public folder mailboxes have copies for high availability.


If one of the mailboxes fails, a passive copy will automatically take over.


Client access and geo scale

Clients connect to the datacenter through their closest Client Access Sever which will proxy them to the public folder mailbox with their home hierarchy. Access to public folder mailboxes is routed within the high bandwidth corporate network or Office 365.

Client home hierarchies are equally distributed across all public folder mailboxes for load balancing. If needed admins can override this per user setting to optimize for proximity.

This optimizes for proximity to the client as well as for partitioning access load to any given copy of the hierarchy.

Client access flow across datacenters

A client in Europe connects to the closest datacenter to his location (usually that datacenter will be hosted in the same geography as the user but let’s say for a moment the user is travelling).

The Client Access Server will reroute him to the public folder mailbox with his home hierarchy. The connection will be made within the datacenter network avoiding slow client connections to remote datacenters over the Internet.


When that client accesses the ‘Finance’ folder, he will be redirected to the mailbox hosting that content.


Writing to the public folder hierarchy

There is only one writeable copy of the hierarchy within a public folder deployment. Writes against the hierarchy (e.g. new folder) will always be performed against that writeable copy.

In the case that the mailbox hosting the writeable copy fails, the writeable hierarchy role will automatically fail over to a passive mailbox copy in the DAG.

What is the difference between site mailboxes, shared mailboxes and public folders?

Site mailboxes

For groups of people that are working together on a shared set of deliverables. They want to keep important emails and documents in one place.

The content is scoped to a particular project that a small team is working on. As such, all content in that mailbox is highly relevant to the team members.

User will not see a site mailbox in their Outlook client unless they are an owner or member of that site mailbox.

Shared mailboxes

A group of people is working on behalf of a virtual entity (e.g. They are triaging incoming emails against a shared inbox and responding on behalf of the virtual entity.

Integrated document collaboration is not a requirement for this scenario.

Users will usually only do this for one shared mailbox and the mailbox is added manually to the user’s Outlook profile.

Public folders

Public folders hold the full body of shared email knowledge in an organization.

Public folders are a great technology for distribution group (DG) archiving. A public folder can be mail enabled and added to the DG. Emails that are sent to the DG will be automatically added to the public folder for later reference.

With the new Office public folders are now also available in Office 365.

Distribution groups

Distribution groups are not actually a shared store in Exchange. They are rather a way for sending emails to a defined set of people such that emails are delivered to those users’ inboxes for triage.


We redesigned modern public folders and moved them to a new architecture that scales well into the future. Public folders are now also available in Office 365.

Public folders support the same high availability strategies as regular mailboxes. They are massively scalable by branching content out into a new mailbox. Building on the mailbox infrastructure r educes storage cost for customers.

Stay tuned

This was just a quick overview of some of the important changes in modern public folders. This blog will have more detailed technical posts on public folders coming soon.

Some of the topics we will cover are:

  • Operating Modern Public Folders
  • Managing Modern Public Folders
  • Modern Public Folders for developers
  • Public Folders in Office 365

Alfons Staerk, Nikhil Aggarwal

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Feedback mentioned are very much true & taken care by Exchange 2013.

Looking forward for the deep drive with PF upcoming articles.

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Public Folders within O365 will allow many more customers to think about the hosted server but I'm interested in scale in terms of actual amounts of data. For example, can a company with terabytes of public folder data realistically look to move all this to O365? Are there any publicised limits to the new architecture?



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Thank you.  We really needed Public Folders for our clients.  Can't wait to test it out.  Again thank you.

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What is the status of the Organizational Forms Library?


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Matt, currently O365 has a maximum limit of 100 public folder mailboxes per tenant, which at 25GB per mailbox puts the technical cap on data size at 2.5TB. Realistically, because it's not possible to fill each of the mailboxes perfectly, you should be able to put about 1TB data before running out of quota. There is no such limit for on-premise deployments though.

Doug, Organizational forms Library continues to be supported in Exchange 2013. All Organizational Forms are migrated to Exchange 2013 during the public folder migration process.

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New Public Folders in Exchange 2013 require Office 2013 Outlook client.

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David, that's not the case. Modern public folders are accessible via Outlook 2007 and Outlook 2010 as well.

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Hi Nikhil,

Sorry if I am wrong I have not had any time to play around with Exchange 2013 yet.

But I have attended Microsoft Ignite in Berlin this fall and it was explained to us that new PF required Office 2013 (Outlook 2013). Is the functionality the same for Outlook 2007, 2010 and 2013 or are they any differences?


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Can anyone clarify which version of O365 must you have to get the public folder capability? Must you have plans E3 or E4 that have e-mail archiving or will all the E plans have this function?

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Awesome Change over in Architecture - It will meet almost all the expectations.

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highlights another decision mistake by microsoft and demonstrates the failure of sharepoint as to how it was meant to replace public folders..

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Confused about which direction to take MS? Even a little???

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Unfortunately the last two comments lack detail to provide a more specific answer.

SharePoint is a great solution for document collaboration that is widely adopted. Public Folders provide a organization-wide shared store for emails. They serve different customer scenarios and needs.

We do support Public Folders and made significant investments to bring them onto a modern architecture that supports a scalable infrastructure both on-premises as well as in the service.

Hope that helps a little for clarification.

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Good work in terms of architecture. Hoping migration is not that difficult.

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I have a client migrating to O365 with Organizational Forms in their current Exchange 2003 server. They want to move that content to shared mailboxes on O365. I know SharePoint would be a better fit but I am new to the project and don't know the details yet. After reading this post, it seems public folders may be an option too. In any case, do you have any guidance on migrating Organizational Forms to O365?


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