ns3.isc.org has no MX records.
Question and response
Question
dig ns3.isc.org. MX
Answer
The response has no answer records.
Authority
isc.org. 3600 SOA ns-int.isc.org. hostmaster.isc.org. 2026022093 7200 3600 24796800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0