mx15.migadu.com has no CNAME records.
Question and response
Question
dig mx15.migadu.com. NS
Answer
The response has no answer records.
Authority
migadu.com. 3600 SOA ns0.migadu.com. hostmaster.migadu.com. 1771958132 1200 900 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0