mailer.upcloud.com has no CNAME records.
Question and response
Question
dig mailer.upcloud.com. NS
Answer
The response has no answer records.
Authority
upcloud.com. 1266 SOA dns1.upcloud.com. dns.cloudflare.com. 2396888040 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0