Apex domains cannot have CNAME records.
Question and response
Question
dig sfdc-ei37dk.mta.salesforce.com. NS
Answer
sfdc-ei37dk.mta.salesforce.com. 5 NS dns04.salesforce.com.
sfdc-ei37dk.mta.salesforce.com. 5 NS dns01.salesforce.com.
sfdc-ei37dk.mta.salesforce.com. 5 NS dns06.salesforce.com.
sfdc-ei37dk.mta.salesforce.com. 5 NS dns02.salesforce.com.
sfdc-ei37dk.mta.salesforce.com. 5 NS dns03.salesforce.com.
sfdc-ei37dk.mta.salesforce.com. 5 NS dns05.salesforce.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0