outbound-mail6.linode.com has no MX records.
Question and response
Question
dig outbound-mail6.linode.com. MX
Answer
The response has no answer records.
Authority
linode.com. 86400 SOA a1-249.akam.net. dns.linode.com. 2026022229 4200 3600 2419200 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0