spf.key-systems.net has no MX records.
Question and response
Question
dig spf.key-systems.net. MX
Answer
The response has no answer records.
Authority
key-systems.net. 180 SOA pdns109.ultradns.com. tech.key-systems.net. 2025124732 3600 600 604800 180
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0