mx.citibesthk.com has no MX records.
Question and response
Question
dig mx.citibesthk.com. MX
Answer
The response has no answer records.
Authority
citibesthk.com. 3600 SOA dns.citibesthk.com. hostmaster.citibesthk.com. 1988239304 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0