mta1a1.sp.actionkit.com has no MX records.
Question and response
Question
dig mta1a1.sp.actionkit.com. MX
Answer
The response has no answer records.
Authority
sp.actionkit.com. 900 SOA ns-1821.awsdns-35.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0