Ensure seamless mail flow in Exchange Hybrid deployments by properly managing TLS certificates. Learn common issues with certificate renewal, key configuration tips, and how to use the Hybrid Configuration Wizard to avoid email disruptions between on-premises Exchange and Exchange Online. Unique :

TLS Certificates in Exchange Hybrid: Common Issues & Fixes

Managing TLS certificates in Exchange hybrid deployments can be tricky. When your on-premises Exchange Server connects with Exchange Online, secure mail routing depends heavily on properly configured certificates. This post dives into common certificate problems and how to solve them.

What’s New in Hybrid Mail Flow?

Hybrid deployment allows seamless mail flow between on-premises Exchange and Exchange Online using the same domain namespace. Typically, the Hybrid Configuration Wizard (HCW) configures certificates automatically. However, renewing third-party certificates often causes mail flow interruptions.

As the Exchange Team explains,

“Running the Hybrid Configuration Wizard configures everything seamlessly, but issues can arise when the on-premises SMTP-assigned certificate is renewed.”

Understanding these nuances is key to avoiding downtime.

Major Updates & Important Considerations

Certificate Renewal Best Practices

• Generate your Certificate Signing Request (CSR) with PrivateKeyExportable set to TRUE.
• Complete the certificate request using Exchange Management Shell or MMC to ensure the private key is exportable.
• Verify that leaf, intermediate, and root certificates are installed correctly to avoid chain errors.

Use the PowerShell command Get-ChildItem -Path "Cert:\LocalMachine\My\" | Test-Certificate to check certificate health.

Handling SMTP Service Assignment

When assigning SMTP service to a renewed certificate, Exchange prompts to overwrite the existing transport certificate. It’s best to keep the self-signed certificate for internal transport services.

“Replacing the self-signed certificate with a third-party one is generally fine, but can cause annual Edge Server resubscription headaches.”

Self-signed certs last 5 years, while third-party certs usually last 1 year. Overwriting can lead to unnecessary maintenance.

How Exchange Selects SMTP Certificates

Exchange looks for certificates matching the TlsCertificateName on connectors. If multiple certs match, it picks the newest by expiry date, regardless of SMTP service assignment.

Therefore, only import certificates when ready to assign SMTP services to avoid confusion.

Steps After Renewing Your Certificate

Always run the Hybrid Configuration Wizard after renewing certificates. The latest HCW version can update mail certificates for connectors without altering custom settings.

Use the Update Secure Mail Certificate for Connectors option during the HCW run to keep mail flow smooth.

Backup & Verify Connector Configurations

Before running HCW, back up your send and receive connectors using PowerShell:

Get-SendConnector | Export-Clixml c:\temp\OnPremSendConBkp.xml
Get-ReceiveConnector | Export-Clixml c:\temp\OnPremRcptConBkp.xml

Similarly, back up Exchange Online connectors remotely:

Get-InboundConnector | Export-Clixml c:\temp\EXOInConBkp.xml
Get-OutboundConnector | Export-Clixml c:\temp\EXOOutConBkp.xml

After HCW runs, review logs at C:\Users\\AppData\Roaming\Microsoft\Exchange Hybrid Configuration to spot any unintended changes.

Final Thoughts

Hybrid Exchange mail flow depends heavily on correctly configured TLS certificates. Renewing certificates without following best practices can cause frustrating mail disruptions.

By understanding certificate roles, backing up connector settings, and using the HCW wisely, you can keep your hybrid environment secure and reliable.

Stay proactive and keep those certs in check!

Hybrid mail flow relies on trusted third-party certificates, not self-signed ones, for secure Exchange Online communication.

Always set ‘PrivateKeyExportable’ to TRUE when generating and importing certificates to avoid key-related issues.

Renewed certificates must be carefully assigned to SMTP services without overwriting essential self-signed transport certificates unnecessarily.

Exchange selects SMTP certificates based on expiry dates and issuer/subject matching, regardless of SMTP service assignment.

Use the Hybrid Configuration Wizard’s “Update Secure Mail Certificate for Connectors” option to safely update certificates without altering custom connector settings.

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