mx4.plusserver.com has no NS records.
NS records for this zone are defined at plusserver.com.
NS records for this zone are defined at plusserver.com.
Question and response
Question
dig mx4.plusserver.com. NS
Answer
The response has no answer records.
Authority
plusserver.com. 900 SOA ns1.plusserver.com. hostmaster.plusserver.com. 2026021109 3600 900 3600000 900
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0