mail6.vultr.com has no CNAME records.
Question and response
Question
dig mail6.vultr.com. NS
Answer
The response has no answer records.
Authority
vultr.com. 300 SOA ns1.choopadns.com. abuse.choopa.com. 0 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0