outbound.mail.linode.com has no NS records.
NS records for this zone are defined at linode.com.
NS records for this zone are defined at linode.com.
Question and response
Question
dig outbound.mail.linode.com. NS
Answer
The response has no answer records.
Authority
linode.com. 86400 SOA a1-249.akam.net. dns.linode.com. 2026022236 4200 3600 2419200 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0