mail.ams.rexum.space has no CNAME records.
Question and response
Question
dig mail.ams.rexum.space. NS
Answer
The response has no answer records.
Authority
rexum.space. 107 SOA elma.ns.cloudflare.com. dns.cloudflare.com. 2398372638 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0