mail.emailsys.net has no MX records.
Question and response
Question
dig mail.rapidmail.de. MX
Answer
The response has no answer records.
Authority
rapidmail.de. 180 SOA ns10.digicertdns.com. dns.digicertdns.com. 2009011399 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0