Three e-mail servers providing one mail domain

[furfoot]

We are currently running a POC for zimbra.

Situation:

2 Exchange servers (exchange1.example.com, exchange2.example.com)
1 Zimbra 7.1 server (zimbra.example.com)

Our policy is to use one domain for our account identities (account@example.com). So we don't want zimbra.example.com to be displayed anywhere in our internal and external communications (from, reply-to addresses, contacts, gal).

I have created the alias example.com in zimbra 7.1 and this enables users to manage their own from and reply-to addresses.

Question 1: I don't want users to be able to have any influence on their from and reply-to address settings. Can I stop them from accessing this functionality somehow?

Question 2: Is it possible to make a domain-wide "policy" stating that all current and newly created accounts in zimbra.example.com are forced by default to use account@example.com as from and reply-to address?