mail4.slfs.com has no NS records.
NS records for this zone are defined at slfs.com.
NS records for this zone are defined at slfs.com.
Question and response
Question
dig mail4.slfs.com. NS
Answer
The response has no answer records.
Authority
slfs.com. 86400 SOA ns1.markmonitor.com. hostmaster.markmonitor.com. 2026022601 86400 3600 2592000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0