ns-mailkit-sg.mailkit.com has no MX records.
Question and response
Question
dig ns-mailkit-sg.mailkit.com. MX
Answer
The response has no answer records.
Authority
mailkit.com. 3600 SOA ns1.mailkit.com. hostmaster.mailkit.com. 2026030501 3600 300 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0