_spf.shell.com has no MX records.
Question and response
Question
dig _spf.shell.com. MX
Answer
The response has no answer records.
Authority
shell.com. 600 SOA dns1.shell.com. dns.shell.com. 2832027 10800 1080 2419200 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0