spf.sophosdmarc.net has no MX records.
Question and response
Question
dig spf.sophosdmarc.net. MX
Answer
The response has no answer records.
Authority
spf.sophosdmarc.net. 1800 SOA lauryn.ns.cloudflare.com. dns.cloudflare.com. 2397897454 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0