dns1.shell.com has no MX records.
Question and response
Question
dig dns1.shell.com. MX
Answer
The response has no answer records.
Authority
shell.com. 600 SOA dns1.shell.com. dns.shell.com. 2830740 10800 1080 2419200 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0