mailers.websitetoolbox.com has no NS records.
NS records for this zone are defined at websitetoolbox.com.
NS records for this zone are defined at websitetoolbox.com.
Question and response
Question
dig mailers.websitetoolbox.com. NS
Answer
The response has no answer records.
Authority
websitetoolbox.com. 900 SOA ns-52.awsdns-06.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0