mail4.sendersrv.com has no NS records.
NS records for this zone are defined at sendersrv.com.
NS records for this zone are defined at sendersrv.com.
Question and response
Question
dig mail4.sendersrv.com. NS
Answer
The response has no answer records.
Authority
sendersrv.com. 1800 SOA edna.ns.cloudflare.com. dns.cloudflare.com. 2399552367 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0