mail.sqldb.us has no CNAME records.
Question and response
Question
dig mail.sqldb.us. NS
Answer
The response has no answer records.
Authority
sqldb.us. 5921 SOA dns.sqldb.us. dns-admin.sqldb.us. 2026010470 10800 3600 1209600 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0