mx4.plusserver.com has no CNAME records.
Question and response
Question
dig mx4.plusserver.com. NS
Answer
The response has no answer records.
Authority
plusserver.com. 687 SOA ns1.plusserver.com. hostmaster.plusserver.com. 2026021109 3600 900 3600000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0