Welcome to the forums,
For your internal domain question-yes that's entirely possible, you could play with:
RestrictPostfixRecipients - ZimbraWiki
There is a RFE open somewhere for a checkbox 'only allow user to send email to same domain addresses'
Edit/found it:
Bug 16671 - 'Internal email only' options in admin control panel Quote:
Make check boxes for:
User can send email outside of his domain
User can recieve mail from outside his domain.
Yes there' are plenty of postfix/whitelist/blacklist methods but this really
needs to be a check box feature.
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combined into:
Bug 9620 - Mail policies and access control for sending to distribution lists