ns1.es.colt.net has no MX records.
Question and response
Question
dig ns1.es.colt.net. MX
Answer
The response has no answer records.
Authority
es.colt.net. 172800 SOA ns0.es.colt.net. dns-requests.colt.net. 2018120600 86400 7200 2592000 172800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0