How do we migrate Exchange public folders to Microsoft 365?

Key facts

• Public folders are supported in Exchange Online, so migration is optional; replacement is a design choice.
• Microsoft provides a batch migration process and scripts for modern public folders from Exchange Server.
• Mail-enabled public folders need their addresses synchronized so mail flow keeps working.
• The final step locks source public folders for a short period while the last sync completes.
• Public folder migration is usually done after mailboxes have moved.

Keep, replace or retire

Public folders often hold a mix of active shared calendars, contact lists, archived email and folders nobody has opened in years. Start with an inventory: folder paths, sizes, item counts, last modified dates, permissions and which folders are mail-enabled. Microsoft's statistics export scripts help here.

Then decide per folder. Active shared mail often fits a shared mailbox. Shared calendars can stay in public folders or move to a group or shared mailbox calendar. Document-style content belongs in SharePoint or Teams. Old, unused folders can be exported and archived or retired with business sign-off.

Prerequisites

Mailboxes are normally migrated first, through a hybrid or cutover migration, because users need to be in Exchange Online to use cloud public folders. Exchange Server versions in current support use modern public folders, which have a direct batch migration path. Very old legacy public folders would need an earlier upgrade or a third-party tool.

Check that the on-premises Exchange servers are on supported, current updates, that the migration endpoint is reachable, and that there are no existing public folder mailboxes in Exchange Online that would conflict.

• Inventory with the Microsoft statistics export scripts.
• Clean up empty and unused folders before migration.
• Fix invalid characters in folder names and aliases.
• Confirm mailbox migrations are complete or nearly complete.
• Plan a short maintenance window for the final lock and sync.

The batch migration process

The Microsoft process generates a mapping of public folders to public folder mailboxes in Exchange Online based on size, creates those mailboxes, then starts a public folder migration batch that copies content while users keep working on-premises. Mail-enabled public folder objects are synchronized to Exchange Online so their addresses resolve.

When the initial sync is complete, you lock the on-premises public folders, run the final incremental sync, verify, and set the organization to use public folders in Exchange Online. The lock period is the only time users cannot access public folders; its length depends on how much changed since the last sync.

Mail flow and permissions

Mail-enabled public folders need care. Their addresses must exist in Exchange Online before cutover, and MX and connector changes must not orphan them. Microsoft's mail public folder sync script handles the directory side; test inbound mail to several folders after cutover.

Permissions migrate with the folders, but review them first. Default and anonymous permissions that were acceptable internally may be broader than you want, and orphaned entries from deleted users should be cleaned up.

Testing and cutover

Test with pilot users in Outlook desktop, Outlook on the web and the new Outlook, checking folder hierarchy, calendars, contacts, posting and send-as permissions. Confirm search works on large folders.

After cutover, keep the on-premises copy locked and unchanged until the business confirms content is complete, then remove it as part of Exchange Server decommissioning. Document where each replaced folder went so users can find it.

How Accred Consulting approaches it

Accred Consulting handles public folders as a defined workstream within Exchange Online migrations. Discovery inventories folders, sizes, mail-enabled addresses, permissions and usage, and the proposal sets scope, fixed price and timeline. The US-based specialists assigned to your engagement agree a keep, replace or retire plan with you, run the Microsoft batch migration, sync mail-enabled folders, move replaced folders to shared mailboxes or SharePoint, and test with pilot users before the final lock. In one Exchange Server exit, an inventory of apps relaying mail prevented an outage. Handover includes documentation and knowledge transfer.

Track record: 475+ client projects, 2,000+ migration workstreams and 210,000+ users migrated by a US-based team. Company facts · Case studies

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