smtp.migadu.com has no CNAME records.
Question and response
Question
dig smtp.migadu.com. NS
Answer
The response has no answer records.
Authority
migadu.com. 3600 SOA ns0.migadu.com. hostmaster.migadu.com. 1772726735 1200 900 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0