ns.eu2.stamul.com has no MX records.
Question and response
Question
dig ns.eu2.stamul.com. MX
Answer
The response has no answer records.
Authority
stamul.com. 600 SOA ns.us4.stamul.com. team.stamul.com. 1497997875 360 3600 604800 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0