DNS domains problem

hi,

I have an issue with my Bind DNS configuration.

Lets say my offical external domain is example.com and we have mail and ftp server running with a webhosting comapny and our ftp host ie ftp.example.com has dynammic ip.

Now i configured a DNS server in our lan for local resolving purpose.
zone name is example.com and it resolves local machines like

machine1.example.com = 192.168.0.1 so on.

problem comes when I try to lookup ftp.example.com since it is an external ip it doesnt resolve. All other exteranl domains are resolved as i put forwarders clause in named.conf pointing to ISP's dns ip.

Somebody help me to find out what the changes i should do?

thank you
lhrt