mail6.six-group.com has no MX records.
Question and response
Question
dig mail6.six-group.com. MX
Answer
The response has no answer records.
Authority
six-group.com. 300 SOA a1-232.akam.net. dnsadmin.six-group.com. 2012222666 3600 360 2592000 300
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0