error：
用客戶端能正常登錄收發郵件，但用域名登錄報錯，
nslookup結果如下：
C:\Documents and Settings\l**>nslookup mail.m.cn
DNS request timed out.
timeout was 2 seconds.
*** Can't find server name for address 192.168.3.7: Timed out
*** Default servers are not available
Server: UnKnown
Address: 192.168.3.7
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Request to UnKnown timed-out
解決：
關閉iptables或者設置開放53端口後ok
iptables -A INPUT -p udp --dport 53 -j ACCEPT iptables配置說明 https://help.ubuntu.com/community/IptablesHowTo