mail.vultrusercontent.com has no NS records.
NS records for this zone are defined at vultrusercontent.com.
NS records for this zone are defined at vultrusercontent.com.
Question and response
Question
dig mail.vultrusercontent.com. NS
Answer
The response has no answer records.
Authority
vultrusercontent.com. 900 SOA ns1.vultr.com. dnsadm.choopa.com. 0 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0