rb.steadfastdns.net has no MX records.
Question and response
Question
dig rb.steadfastdns.net. MX
Answer
The response has no answer records.
Authority
steadfastdns.net. 300 SOA r0.steadfastdns.net. root.steadfast.net. 2025030401 86400 86400 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0