mailserver.libware.pt has no CNAME records.
Question and response
Question
dig mailserver.libware.pt. NS
Answer
The response has no answer records.
Authority
libware.pt. 1782 SOA ns2.no-ip.com. hostmaster.no-ip.com. 2015030523 10800 1800 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0