mail.e-strefa.net has no CNAME records.
Question and response
Question
dig mail.e-strefa.net. NS
Answer
The response has no answer records.
Authority
e-strefa.net. 10800 SOA ns1.e-strefa.net. admin.sitel.net.pl. 2014053000 3600 14400 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0