spf3.npl.co.uk has no MX records.
Question and response
Question
dig spf3.npl.co.uk. MX
Answer
The response has no answer records.
Authority
npl.co.uk. 600 SOA ednssvr1.npl.co.uk. postmaster.npl.co.uk. 2026012201 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0