dsn.tefdigital.com has no MX records.
Question and response
Question
dig dsn.tefdigital.com. MX
Answer
The response has no answer records.
Authority
tefdigital.com. 86400 SOA ns10.servicio-online.net. hostmaster.ns10.servicio-online.net. 1771974566 14400 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0