smtp3.cdk.com has no MX records.
Question and response
Question
dig smtp3.cdk.com. MX
Answer
The response has no answer records.
Authority
cdk.com. 300 SOA ns.las.us.cdk.com. domain-admin.cobaltgroup.com. 2026021701 3600 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0