sub531.htc.net has no MX records.
Question and response
Question
dig sub531.htc.net. MX
Answer
The response has no answer records.
Authority
htc.net. 900 SOA ns.htc.net. postmaster.ns.htc.net. 2026011201 7200 3600 1209600 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0