a.mx.colocall.net has no MX records.
Question and response
Question
dig a.mx.colocall.net. MX
Answer
The response has no answer records.
Authority
colocall.net. 3600 SOA ns1.colocall.net. hostmaster.colocall.net. 2026021600 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0