mx.mastwall.com has no CNAME records.
Question and response
Question
dig mx.mastwall.com. NS
Answer
The response has no answer records.
Authority
mastwall.com. 3136 SOA dns.mastwall.com. hostmaster.mastwall.com. 1134478552 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0