o3.sg.calendly.com has no MX records.
Question and response
Question
dig o3.sg.calendly.com. MX
Answer
The response has no answer records.
Authority
calendly.com. 1800 SOA hope.ns.cloudflare.com. dns.cloudflare.com. 2397010917 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0