smtp1.aptum.com has no CNAME records.
Question and response
Question
dig smtp1.aptum.com. NS
Answer
The response has no answer records.
Authority
aptum.com. 1800 SOA art.ns.cloudflare.com. dns.cloudflare.com. 2397362270 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0