Exchange Online Autodiscover CNAME Transition: Unresolved TLS Validation After DNS Misconfiguration

Goal

Verify that an Autodiscover CNAME record resolves to autodiscover.outlook.com and that the TLS certificate presented by Exchange Online matches the custom domain used by Outlook clients.

Uncertainty

When a CNAME exists but points to a non‑Microsoft host, Exchange Online may still return a DigiCert certificate containing the custom domain. Outlook clients, however, reject the connection due to a hostname mismatch. Microsoft has not yet published automated remediation guidance for this scenario.

Questions

• What diagnostic steps can reliably detect that a CNAME record is pointing to a non‑Microsoft host while still allowing Exchange Online to present a valid certificate?
• How can administrators programmatically verify that the certificate’s Subject Alternative Name matches the domain referenced in the CNAME chain?
• What best‑practice remediation workflow exists for correcting such misconfigurations without disrupting existing mail flow?