IPv6.CARNet.hr has no MX records.
Question and response
Question
dig IPv6.CARNet.hr. MX
Answer
The response has no answer records.
Authority
IPv6.CARNet.hr. 14400 SOA ns1.IPv6.CARNet.hr. ipv6hostmaster.CARNet.hr. 2012071301 10800 3600 2419200 14400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0