What Happens to Shared Calendars After Exchange Hybrid Migration?

Applies to: Exchange Server SE / 2019 / 2016 | Exchange Online | Hybrid

In Exchange Hybrid environments, shared calendar behavior after mailbox migration is often misunderstood. Calendar access does not automatically fail when a mailbox moves to Exchange Online, but the result depends on the permission type, how it was assigned, and the hybrid configuration. This article walks through a controlled test and shows the behavior observed before and after the move.

In This Article

• 1. Scenario
• 2. Step 1 – Test Default Behavior (On-Prem → On-Prem)
• 3. Step 2 – Check Existing Permissions
• 4. Step 3 – Assign Explicit Permission
• 5. Step 4 – Verify Access
• 6. Step 5 – Migrate Mailbox to Exchange Online
• 7. Step 6 – Test After Migration (Both Directions)
  • 7.1 Test 1 – On-Prem → Exchange Online
  • 7.2 Test 2 – Exchange Online → On-Prem
• 8. What This Test Demonstrated
• 9. If Calendar Access Fails After the Move
• 10. Key Takeaways

1. Scenario

We use two users:

• buser01@onpremx.com
• bmuser01@onpremx.com

Initial state:

• Both mailboxes are on on-prem Exchange
• Users can access each other’s calendars

2. Step 1 – Test Default Behavior (On-Prem → On-Prem)

Without assigning any permissions:

• Add calendar in Outlook
• Calendar is added successfully
• Only free/busy information is visible
• No subject or details

This is expected.

In this test, the Default calendar permission was:

AvailabilityOnly

AvailabilityOnly

3. Step 2 – Check Existing Permissions

Verify current permissions:

Get-MailboxFolderPermission bmuser01:\Calendar
Get-MailboxFolderPermission buser01:\Calendar

Get-MailboxFolderPermission bmuser01:\Calendar
Get-MailboxFolderPermission buser01:\Calendar

Typical result:

No explicit user permissions are configured.

4. Step 3 – Assign Explicit Permission

Grant Reviewer access:

Add-MailboxFolderPermission bmuser01:\Calendar -User buser01 -AccessRights Reviewer

Add-MailboxFolderPermission buser01:\Calendar -User bmuser01 -AccessRights Reviewer

Add-MailboxFolderPermission bmuser01:\Calendar -User buser01 -AccessRights Reviewer

Add-MailboxFolderPermission buser01:\Calendar -User bmuser01 -AccessRights Reviewer

5. Step 4 – Verify Access

After assigning permission:

• Calendar opens successfully
• Subject and details are visible

6. Step 5 – Migrate Mailbox to Exchange Online

Move bmuser01 to Exchange Online using Hybrid migration (batch or remote move).

After migration:

• buser01 → on-prem
• bmuser01 → Exchange Online

7. Step 6 – Test After Migration (Both Directions)

After the migration, test calendar access from both sides.

7.1 Test 1 – On-Prem → Exchange Online

User: buser01 (on-prem)
Target: bmuser01 (Exchange Online)

• Add calendar from Outlook
• Calendar opens successfully
• Full details are visible

7.2 Test 2 – Exchange Online → On-Prem

User: bmuser01 (Exchange Online)
Target: buser01 (on-prem)

• Add calendar from Outlook or OWA
• Calendar opens successfully
• Full details are visible

8. What This Test Demonstrated

The important distinction in this test is between free/busy visibility provided by the Default calendar permission and an explicit folder permission. In this environment, the Default entry was AvailabilityOnly, so only free/busy information was visible. After explicit Reviewer permissions were assigned, both users could see calendar details.

• The explicit Reviewer permissions were configured before the mailbox move.
• After bmuser01 moved to Exchange Online, the on-premises user could still open that calendar with details.
• The Exchange Online user could also open the remaining on-premises user’s calendar with details.
• No calendar permission had to be recreated for this controlled migration test.

This is the result observed in the specific Exchange Hybrid scenario tested above, not a guarantee for every cross-premises calendar configuration. Microsoft documents limitations for mailbox and folder permissions in hybrid environments and recommends moving mailboxes that participate in folder-permission relationships together. If access fails after a move, verify the folder permission itself and the failing cross-premises direction before recreating permissions.

9. If Calendar Access Fails After the Move

Before removing and recreating calendar permissions, I would first confirm what actually changed. Start by checking the folder permission on the calendar and compare it with the permission level that existed before migration.

Get-MailboxFolderPermission user@contoso.com:\Calendar

• Confirm whether the expected user is still listed on the Calendar folder.
• Distinguish default AvailabilityOnly free/busy visibility from an explicit permission such as Reviewer.
• Test access in both directions when one mailbox is in Exchange Online and the other remains on-premises.
• Only recreate the permission after confirming that the existing folder permission is missing or incorrect.

This keeps a migration-related access problem from being mistaken for a permission problem without evidence.

10. Key Takeaways

• In this controlled test, the explicit Reviewer calendar permissions continued to work after one mailbox was moved to Exchange Online.
• Explicit folder permissions can migrate with a mailbox, but cross-premises permission behavior has documented limitations and should not be treated as universally supported in every topology.
• If calendar access fails after a move, verify the existing folder permission and the affected cross-premises direction before removing and recreating permissions.

References

• Permissions in Exchange hybrid deployments – Microsoft Learn
• Exchange Server hybrid deployments – Microsoft Learn
• Add-MailboxFolderPermission – Microsoft Learn

Ceyhun Kirmizitas

Cloud and infrastructure professional with nearly two decades of experience in enterprise IT environments, spanning public cloud, private cloud, and hybrid architectures.