_mail.glesys.com has no CNAME records.
Question and response
Question
dig _mail.glesys.com. NS
Answer
The response has no answer records.
Authority
glesys.com. 3600 SOA ns1.namesystem.se. registry.glesys.se. 2014071141 14400 2700 1814400 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0