mail-zrh1.exoscale.com has no NS records.
NS records for this zone are defined at exoscale.com.
NS records for this zone are defined at exoscale.com.
Question and response
Question
dig mail-zrh1.exoscale.com. NS
Answer
The response has no answer records.
Authority
exoscale.com. 3600 SOA ns1.exoscale.io. admin.dnsimple.com. 1518553154 86400 7200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0