sender0001.mailingboss.net has no MX records.
Question and response
Question
dig sender0001.mailingboss.net. MX
Answer
The response has no answer records.
Authority
mailingboss.net. 1800 SOA devin.ns.cloudflare.com. dns.cloudflare.com. 2395429792 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0