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

To resolve TLS validation failures after a DNS misconfiguration, you must ensure the Autodiscover CNAME record points strictly to autodiscover.outlook.com. When a CNAME points to a non-Microsoft host, Outlook clients will attempt a TLS handshake with that incorrect host. Even if that host presents a certificate containing your custom domain, the client will reject the connection because the hostname in the URL does not match.

Diagnostic Steps

To detect if a CNAME is pointing to a non-Microsoft host while Exchange Online still serves a valid certificate, follow these steps:

• Verify Resolution: Use nslookup -type=cn autodiscover.domain.com. If the target is not autodiscover.outlook.com, the record is misconfigured.
• Inspect Handshake: Use openssl s_client -connect autodiscover.yourdomain.com:443 to view the certificate being returned. Look for the Subject Alternative Name (SAN).
• Check Client Behavior: Use the Outlook Autodiscover Test Tool (APIM). If the test fails at the "HTTPS" stage with a certificate mismatch, the CNAME target is likely the culprit.

Programmatic Verification

Administrators can programmatically verify that the certificate's SAN matches the domain using PowerShell. This script ensures the certificate presented by the resolved endpoint matches the expected domain:

$domain = "autodiscover.yourdomain.com"
$connection = Test-NetConnection -ComputerName $domain -Port 443 -InformationLevel
$stream = ([System.Net.Sockets.TcpClient]$connection.TcpClientObject).GetStream()
$sslStream = New-Object System.Net.Security.SslStream($stream, $false)
$sslStream.AuthenticateAsAsClient($domain)
$cert = $sslStream.RemoteCertificateObject as System.Security.Cryptography.X509ificates.X509Certificate
$san = $cert.Extensions["2.5.10.17"].Value
$san -like "*$domain*"

Remediation Workflow

To correct the misconfiguration without disrupting existing mail flow, follow this workflow:

1. Update DNS Record: Change the Autodiscover CNAME to point to autodiscover.outlook.com.
2. Wait for Propagation: Allow for the TTL (Time to Live) to expire so that client-side caches are updated globally.
3. Clear Client Cache: On affected machines, flush the DNS cache using ipconfig /flushdns and clear the Outlook Autodiscover cache located in the user's AppData.
4. Verify: Re-run the diagnostic steps above to confirm the TLS handshake is now valid against the Microsoft-hosted certificate.

Note: Mail flow (SMTP/IMAP) is generally unaffected by Autodiscover CNAME records, as these are primarily used for client-side configuration.