Apex domains cannot have CNAME records.
Question and response
Question
dig postmarkapp.com. NS
Answer
postmarkapp.com. 5 NS ns-1262.awsdns-29.org.
postmarkapp.com. 5 NS ns-1657.awsdns-15.co.uk.
postmarkapp.com. 5 NS ns-229.awsdns-28.com.
postmarkapp.com. 5 NS ns-721.awsdns-26.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0