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