rhsmtp05.hkcsl.com has no MX records.
Question and response
Question
dig rhsmtp05.hkcsl.com. MX
Answer
The response has no answer records.
Authority
hkcsl.com. 38400 SOA ns3.hkcsl.com. dns.hkcsl.com. 2026010701 10800 3600 604800 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0