mail1.wpengine.com has no NS records.
NS records for this zone are defined at wpengine.com.
NS records for this zone are defined at wpengine.com.
Question and response
Question
dig mail1.wpengine.com. NS
Answer
The response has no answer records.
Authority
wpengine.com. 1800 SOA jim.ns.cloudflare.com. dns.cloudflare.com. 2397329181 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0