b.ns.3m.com has no MX records.
Question and response
Question
dig b.ns.3m.com. MX
Answer
The response has no answer records.
Authority
3m.com. 1800 SOA a.ns.3m.com. dns.cloudflare.com. 2397101002 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0