sigma1.postal.cyso.net has no MX records.
Question and response
Question
dig sigma1.postal.cyso.net. MX
Answer
The response has no answer records.
Authority
postal.cyso.net. 300 SOA ns1.cyso.nl. dnsadmin.cysonet.com. 1772064000 16384 2048 1048576 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0