b.mx.sardinasystems.com has no MX records.
Question and response
Question
dig b.mx.sardinasystems.com. MX
Answer
The response has no answer records.
Authority
sardinasystems.com. 60 SOA a.ns.sardinasystems.com. hostmaster.sardinasystems.com. 1771157890 300 60 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0