_spfextra.slack.com has no CNAME records.
Question and response
Question
dig _spfextra.slack.com. NS
Answer
The response has no answer records.
Authority
slack.com. 300 SOA ns-1493.awsdns-58.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0