in21.exim.smtpcorp.com has no MX records.
Question and response
Question
dig in21.exim.smtpcorp.com. MX
Answer
The response has no answer records.
Authority
smtpcorp.com. 1800 SOA ada.ns.cloudflare.com. dns.cloudflare.com. 2397538372 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0