mx.ams1.isc.org has no CNAME records.
Question and response
Question
dig mx.ams1.isc.org. NS
Answer
The response has no answer records.
Authority
ams1.isc.org. 3600 SOA ns-int.isc.org. postmaster.isc.org. 2026011082 7200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0