mx1.simplelists.com has no CNAME records.
Question and response
Question
dig mx1.simplelists.com. NS
Answer
The response has no answer records.
Authority
simplelists.com. 3586 SOA ns1.simplelists.com. hostmaster.simplelists.com. 1 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0