sandbox.billingplatform.com has no MX records.
Question and response
Question
dig sandbox.billingplatform.com. MX
Answer
The response has no answer records.
Authority
billingplatform.com. 600 SOA pdns01.domaincontrol.com. dns.jomax.net. 2026012700 28800 7200 604800 600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0