mail.otrs.com has no NS records.
NS records for this zone are defined at otrs.com.
NS records for this zone are defined at otrs.com.
Question and response
Question
dig mail.otrs.com. NS
Answer
The response has no answer records.
Authority
otrs.com. 3600 SOA ns1.your-server.de. postmaster.your-server.de. 2026021900 3600 1800 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0