rmm.sisc.com has no MX records.
Question and response
Question
dig rmm.sisc.com. MX
Answer
The response has no answer records.
Authority
sisc.com. 1800 SOA kirk.ns.cloudflare.com. dns.cloudflare.com. 2397725882 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0