mx1.improvmx.com has no MX records.
Question and response
Question
dig mx1.improvmx.com. MX
Answer
The response has no answer records.
Authority
improvmx.com. 1800 SOA logan.ns.cloudflare.com. dns.cloudflare.com. 2396577174 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0