what wrong with my "split dns" ?

[lananhbin]

i am in trouble with my zcs.i can receive email well but i cannt send email smoothly because sometimes my zcs warms

host or domain name not found. name service error for name=xxx type=a: host not found, try again (1 messages)

although i'm sure that the both user and domain exist. in some case, i requeue the deferred emails and they are sent fine but sometimes they arent( but if i reboot my zcs, they "fly" well T_T ). i toptally confuse . so i post my configured file here and i hope you guys help me out. i checked my internet connection but it's ok

$TTL 38400
mydomain.com. IN SOA z.mydomain.com. admin.mydomain.com. (
10118 ; Serial
43200 ; Refresh
3600 ; Retry
3600000 ; Expire
2592000 ) ; Minimum
; Define the nameservers and the mail server
mydomain.com. IN NS z.mydomain.com.
z.mydomain.com. IN A 192.168.1.2
mail.mydomain.com. IN A 192.168.1.2
mydomain.com. IN MX 10 z.mydomain.com.

cat /etc/hosts

127.0.0.1 localhost.localdomain localhost
192.168.1.2 z.mydomain.com z

thanks in advance !