ds1-b2.stl1.constant.com has no MX records.
Question and response
Question
dig ds1-b2.stl1.constant.com. MX
Answer
The response has no answer records.
Authority
constant.com. 3600 SOA ns1.choopadns.com. abuse.choopa.com. 0 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0