[Question] How to prevent sending to public emails

[sonnv]

Hello,

I'm sorry if I post this question in wrong box.

I have a LAN and installed Zimbra Free version in one of my RH5 servers. In Zimbra Admin panel, I configured an internal email domain like @internalmail.com add some users uses this domain (ex: user1@internalmail.com, user2@...)

I'm trying to implement a rule that this Zimbra mail server only be used by internal user accounts of @internalmail.com (send/receive between the internal accts only) and the users can not send out email to any public mail like gmail, yahoo, microsoft.com, ..

Anyone help pointing me out this configuration?

Many thanks.