mx1.cs.getresponse.com has no NS records.
NS records for this zone are defined at getresponse.com.
NS records for this zone are defined at getresponse.com.
Question and response
Question
dig mx1.cs.getresponse.com. NS
Answer
The response has no answer records.
Authority
getresponse.com. 180 SOA ns0.dnsmadeeasy.com. dns.dnsmadeeasy.com. 2020014245 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0