Sharing and collaboration

Federated sharing

Federation refers to the underlying trust infrastructure that supports federated sharing, a method for Microsoft Exchange Online users to share free/busy calendar data and contact information with recipients in other external federated organizations or with users that have internet access. These include organizations that are also hosted by Exchange Online, or external Microsoft Exchange Server 2010 or Exchange Server 2013 organizations. Using organization relationships and sharing policies, Exchange Online administrators can enable users to send calendar-sharing invitations from Microsoft Outlook on the web or Microsoft Outlook 2010 or later.


External Exchange 2010 and Exchange 2013 organizations must configure a federation trust with the Microsoft Federation Gateway as part of configuring federated sharing. Exchange Online organizations don't have to configure a federation trust—the federation trust with the Microsoft Federation Gateway is automatically created when the Microsoft 365 organization is created.

Exchange Online organizations must configure either an organization relationship or a sharing policy to enable federated sharing.

Sharing of global access lists (GALs) or moving user mailboxes between Exchange Online organizations in different Microsoft plans is not supported in federated sharing.

For more information about federated sharing, see Sharing in Exchange Online.

Site mailboxes

Email and documents are traditionally kept in two unique and separate data repositories. Most teams collaborate by using both email and documents. The challenge is that email and documents are accessed by using different clients. This usually results in a reduction in user productivity and a degraded user experience.

The site mailbox is a new concept in Exchange 2013 that attempts to solve this problem. Site mailboxes improve collaboration and user productivity by using the same client interface to allow access to both Microsoft SharePoint 2013 documents and Exchange email. A site mailbox functionally consists of SharePoint 2013 site membership (owners and members), shared storage through an Exchange 2013 mailbox for email messages and a SharePoint 2013 site for documents, and a management interface that addresses provisioning and life cycle needs.


Your plan must include SharePoint. Site mailboxes require that users have both SharePoint and Exchange licenses.

For more information about site mailboxes, see Site Mailboxes.

Public folders

Public folders in Exchange Online have been modernized to take advantage of the existing high availability and storage technologies of the mailbox database. The public folder architecture uses specially designed mailboxes to store both the hierarchy and the public folder content. This means that there's no longer a separate public folder database. Public folder replication now uses the continuous replication model. High availability for the hierarchy and content mailboxes is provided by a database availability group (DAG) in the data center. In Exchange Online, you are limited to 1000 public folder mailboxes. Each public folder mailbox also has a maximum storage size. For more information, see the "Mailbox folder limits" section in Exchange Online limits. Public folder mailboxes have the same message, recipient, and capacity alert limits as regular mailboxes. For more information, see Recipients.

For more information about public folders, see Public Folders.

Group and shared mailboxes

Group and shared mailboxes make it easy for a specific group of people to monitor and send email from a common account, like public email addresses (for example, or When a person in the group replies to a message sent to the shared mailbox, the email appears to be from the Shared mailbox, not from the individual user.

Typically, group or shared mailboxes don't require a separate user license. However, to enable In-Place Archive for a group or shared mailbox, you must assign an Exchange Online Plan 1 or Exchange Online Plan 2 license to it. After the license is assigned, the mailbox size increases to that of the licensed plan. To put a shared mailbox on In-Place Hold, you must assign an Exchange Online Plan 2 license to it. Please note that group mailboxes cannot be assigned at this time but should be accounted for in your total licenses.

In-Place Archive can only be used to archive mail for a single user or entity (such as a Shared mailbox) for which a license has been applied. Using an In-Place Archive as a means of storing mail from multiple users or entities is prohibited. For example, an IT administrator can't create a shared mailbox and have users copy it (through the Cc or Bcc field, or through a transport rule) for the explicit purpose of archiving. Note that a shared mailbox that multiple people use does not actually store email for those individual users. Multiple users have access, and they send email as the shared mailbox. Therefore, the only emails stored in the shared mailbox are those sent to or from it, as the shared mailbox.

Feature availability

To view feature availability across plans, standalone options, and on-premises solutions, see Exchange Online service description.