Troubleshooting Exchange Online Users Unable to View On-Premises Calendar Information

Organizations with an Exchange Hybrid configuration often rely on seamless collaboration between Exchange Online and Exchange On-Premises users. However, calendar availability issues can arise, preventing Exchange Online users from viewing Free/Busy (F/B) information for on-premises calendars. This guide explains why this issue occurs and provides step-by-step solutions to resolve it.

Understanding the Issue

In a hybrid Exchange environment, calendar availability information is shared through the Availability Service using Free/Busy lookup. When Exchange Online users cannot access calendar details of on-premises users, it often points to a misconfiguration in:

• Federation Trust
• Organization Relationships
• Authentication protocols (OAuth or Legacy Authentication)
• Autodiscover settings
• Firewall or network access rules

Addressing these factors ensures a seamless hybrid environment where users can collaborate effectively.

Key Causes of Calendar Availability Issues

1. Federation Trust Misconfiguration
   The Federation Trust that enables secure sharing between Exchange Online and On-Premises may be incomplete.
2. Organization Relationship Errors
   The settings governing sharing policies between environments may be missing or incorrectly configured.
3. Authentication Failures
   OAuth authentication, required for hybrid environments, might not be enabled or functioning properly.
4. Autodiscover Resolution Failures
   Autodiscover cannot resolve or connect to on-premises endpoints.
5. Network or Firewall Restrictions
   Firewalls may block Exchange Web Services (EWS) or Autodiscover traffic.
6. Service Principal Name (SPN) Issues
   SPNs required for OAuth authentication may be missing or misconfigured.

Step-by-Step Solution

1. Verify Organization Relationship Configuration

Check Organization Relationship settings to confirm sharing policies:

Get-OrganizationRelationship | Format-List

Key parameters to verify:

• TargetApplicationURI – Matches the Autodiscover endpoint for on-premises servers.
• TargetSharingEpr – Points to the on-premises EWS URL, e.g., https://mail.contoso.com/EWS/Exchange.asmx.
• Enabled – Should be set to True.

Update the settings if incorrect:

Set-OrganizationRelationship -Identity “OnPremisesRelationship” -TargetSharingEpr “https://mail.contoso.com/EWS/Exchange.asmx”

2. Validate Federation Trust

Confirm the Federation Trust configuration:

Get-FederationTrust | Format-List

If missing or invalid, recreate the Federation Trust using the Exchange Admin Center (EAC) or PowerShell.

3. Enable and Test OAuth Authentication

Ensure OAuth authentication is enabled for secure connections:

Get-IntraOrganizationConnector | Format-List

Recreate or update the connector if required:

New-IntraOrganizationConnector -Name “HybridConnector” -TargetAddressDomains contoso.com -DiscoveryEndpoint “https://outlook.office365.com/autodiscover/autodiscover.svc”

4. Check Autodiscover Configuration

Use the Microsoft Remote Connectivity Analyzer at testconnectivity.microsoft.com to verify Autodiscover functionality.

Also, run the following PowerShell command:

Test-OutlookWebServices -Identity user@domain.com

If errors occur:

• Verify DNS records (CNAME or SRV) for Autodiscover.
• Update the Service Connection Point (SCP) using:

Set-ClientAccessServer -Identity CAS01 -AutoDiscoverServiceInternalUri https://mail.contoso.com/autodiscover/autodiscover.xml

5. Test Free/Busy Lookup

Verify Free/Busy lookup between environments:

Test-FederationTrust
Test-OrganizationRelationship -Identity “OnPremisesRelationship”

Review results and resolve any reported errors.

6. Inspect Network and Firewall Rules

Ensure the following rules allow required traffic:

• HTTPS (Port 443) for EWS and Autodiscover endpoints.
• Inbound and outbound traffic to Exchange Online IP ranges.
• Run:

Test-WebServicesConnectivity -MailboxCredential (Get-Credential)

Resolve firewall or proxy blocks based on results.

7. Re-run the Hybrid Configuration Wizard (HCW)

Rerun the HCW to repair hybrid configurations:

1. Download the wizard from the Microsoft 365 Admin Center.
2. Follow prompts to re-establish Federation Trust, OAuth, and connectors.
3. Test calendar sharing again.

Best Practices for Hybrid Exchange Calendar Availability

• Plan Ahead: Ensure proper DNS, firewall, and trust configurations before enabling hybrid setups.
• Monitor Logs: Use message tracking and diagnostic logs to detect early issues.
• Staged Migration: Implement phased migrations to minimize downtime and errors.
• Test Connectivity Regularly: Schedule recurring connectivity tests to maintain performance.
• Engage Experts: Work with trusted partners like Medha Cloud for smooth hybrid deployments.

Why Use Medha Cloud for Exchange Hybrid Support?

Configuring and troubleshooting hybrid Exchange environments can be complex, particularly when integrating calendar availability. Medha Cloud simplifies this process by providing:

• Expert Exchange Hybrid Setup and Management.
• Free/Busy and Calendar Sharing Troubleshooting.
• Security and Authentication Configuration.
• Ongoing Monitoring and Support Services.

Final Thoughts and Next Steps

Calendar availability between Exchange Online and On-Premises is critical for seamless collaboration. While hybrid configurations introduce complexities, following this step-by-step troubleshooting guide can resolve most issues.

For businesses facing persistent Exchange Hybrid challenges, Medha Cloud offers expert migration, configuration, and troubleshooting services to streamline Exchange environments.

For professional assistance, Contact Medha Cloud—your trusted Microsoft Certified Partner. From setup to ongoing support, we make Exchange Hybrid environments work effortlessly for your business.

Reach us at:

• India: +91 93536 44646
• US: +1 646 775 2855
• Website: www.medhacloud.com
• Email: info@medhacloud.com