Public Folders

Manage Exchange Online public folders for shared access to emails, calendars, contacts, and other items. Public folders provide a traditional way to share information across teams without requiring group membership.

Warning: Microsoft recommends Microsoft 365 Groups and SharePoint for new collaboration needs. Public folders are supported for legacy scenarios and migrations from on-premises Exchange.

Public Folder Hierarchy

Public folders are organized in a hierarchical tree structure:

\ (Root)
├── Company Announcements
├── Departments
│   ├── HR
│   ├── Finance
│   └── IT
├── Projects
│   ├── Project Alpha
│   └── Project Beta
└── Shared Calendars

Public Folder Types

Mail Folders

Store email messages. Can be mail-enabled to receive external email. Common for shared inboxes like support@company.com.

Calendar Folders

Shared calendars for team events, room scheduling, or company-wide events visible to all users.

Contact Folders

Shared address books for department contacts, vendor lists, or customer directories accessible by the team.

Note/Post Folders

Discussion boards and bulletin boards. Users can post and reply to messages in threaded conversations.

Public Folder Mailboxes

Public folders are stored in dedicated mailboxes:

Primary Hierarchy Mailbox

Contains the writable copy of the folder hierarchy. Changes to structure (new folders, renames) happen here first.

Secondary Mailboxes

Additional mailboxes for content storage and load distribution. Each has read-only copy of hierarchy.

Content Mailbox

Each folder’s content is stored in one mailbox. Large folders can be split across mailboxes for performance.

Note: Each public folder mailbox has a 100 GB quota by default. Create additional mailboxes for large deployments.

Permissions

Mail-Enabling Public Folders

Allow public folders to receive email from internal or external senders:

Email Address

Assign an email address (e.g., support@company.com). Mail sent to this address is delivered to the public folder.

Delivery Options

Forward copies to additional recipients. Send on behalf permissions for team members replying as the folder address.

Message Restrictions

Accept from all senders, specific senders only, or require sender authentication to prevent spam.

Creating Public Folders

1. Create Public Folder Mailbox — Must have at least one mailbox before creating folders.
2. Create Folder — Specify name, path, and folder type (mail, calendar, contacts).
3. Set Permissions — Add users/groups with appropriate roles.
4. Mail-Enable (Optional) — Assign email address if folder should receive mail.

Accessing Public Folders

Outlook Desktop

Expand “Public Folders” in folder list. Add favorites for quick access. Full editing capabilities based on permissions.

Outlook Web Access

Right-click Folders, then Add public folder to Favorites. Limited functionality compared to desktop.

Outlook Mobile

Add public folder calendar to shared calendars. Limited support for other folder types.

EWS/Graph API

Programmatic access for custom applications. Full CRUD operations on folder content.

Best Practices

• Consider modern alternatives — Use Microsoft 365 Groups or SharePoint for new collaboration scenarios.
• Organize with logical hierarchy — Use clear folder names and structure. Avoid deeply nested folders.
• Monitor folder sizes — Large folders impact performance. Archive old content regularly.
• Apply least privilege — Grant minimum necessary permissions. Use Reviewer for most users.

API Reference

GET /api/exchange/public-folders List public folders

POST /api/exchange/public-folders Create public folder

GET /api/exchange/public-folder-mailboxes List public folder mailboxes

PUT /api/exchange/public-folders/:id/permissions Update folder permissions

POST /api/exchange/public-folders/:id/mail-enable Mail-enable folder