microsoft exchange 2003

I have TWO main offices on different locations ?.one exchange server 2003 is configured and working fine both on pop3 and OWA from all branches and sub offices ?I have registered live ip where my office web site is hosted all setup was configured at our I.T division
Incoming mail.xyz.com.pk
Outgoing mail.xyz.com.pk
Now managements want to setup a sub I.T room in other office and want their email setup there on LAN so they can access their mail boxes locally so I installed a Domain controller there with dhcp ,Dns and active directory and Microsoft exchange server 2003 on that D.C and name that server xyz1 so now there on local LAN incoming and outgoing servers are
Incoming mail.xyz1.com.pk
Outgoing mail.xyz1.com.pk
This office is connected with our I.T department on a VPN provided by service provider. Mail is working fine on local LAN but I cant send mails to addresses with mail.xyz.com.pk and also not on other domains like yahoo hotmail etc etc please help me to solve this
What configuration I need to do