nessus-outside.retarus.com has no MX records.
Question and response
Question
dig nessus-outside.retarus.com. MX
Answer
The response has no answer records.
Authority
retarus.com. 3600 SOA ns-732.awsdns-27.net. awsdns-hostmaster.amazon.com. 20250602 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0