mx1.open-xchange.com has no CNAME records.
Question and response
Question
dig mx1.open-xchange.com. NS
Answer
The response has no answer records.
Authority
open-xchange.com. 213 SOA dns1.easydns.com. zone.easydns.com. 1769774504 3600 600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0