do-not-use-this.mail4.exim.smtpcorp.com has no MX records.
Question and response
Question
dig do-not-use-this.mail4.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