mail.do.de has no NS records.
NS records for this zone are defined at do.de.
NS records for this zone are defined at do.de.
Question and response
Question
dig mail.do.de. NS
Answer
The response has no answer records.
Authority
do.de. 120 SOA ns1.mainns.de. info.domains.resellerinterface.de. 1774226733 14400 3600 604800 120
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0