mail1.wpengine.com has no CNAME records.
Question and response
Question
dig mail1.wpengine.com. NS
Answer
The response has no answer records.
Authority
wpengine.com. 1705 SOA jim.ns.cloudflare.com. dns.cloudflare.com. 2397358758 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0