mx.runbox.com has no CNAME records.
Question and response
Question
dig mx.runbox.com. NS
Answer
The response has no answer records.
Authority
runbox.com. 3176 SOA dns61.copyleft.no. hostmaster.copyleft.no. 3000010757 14400 3600 1296000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0