smtp.conventioncalendar.com has no CNAME records.
Question and response
Question
dig smtp.conventioncalendar.com. NS
Answer
The response has no answer records.
Authority
conventioncalendar.com. 2888 SOA dns1.swiftrfp.net. admin.swiftrfp.net. 2023011256 3600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0