Apex domains cannot have CNAME records.
Question and response
Question
dig mail.openhouse.org. NS
Answer
mail.openhouse.org. 5 NS ns2.parkingcrew.net.
mail.openhouse.org. 5 NS ns1.parkingcrew.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0