fe.rimail.app has no MX records.
Question and response
Question
dig fe.rimail.app. MX
Answer
The response has no answer records.
Authority
rimail.app. 3601 SOA dns1.registrar-servers.com. hostmaster.registrar-servers.com. 1768955520 43200 3600 604800 3601
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0