denmta01.authorize.net has no MX records.
Question and response
Question
dig denmta01.authorize.net. MX
Answer
The response has no answer records.
Authority
authorize.net. 300 SOA lloyd.ns.cloudflare.com. dns.cloudflare.com. 2019103491 10000 2400 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0