Preventing Windows AD domain name from being used in outbound email

[TrinityEMS]

Hello all,
I'd like to try simplifying our setup a bit. There is some legacy configuration and some cruft I'd like to remove if possible. Here's an overview of our System.

Domains
=================
ADdomain.local
publicdomain.com
zimbra.publicdomain.com

The ADdomain.local is the domain name tied to AD and is used for authentication.
The zimbra.publicdomain.com was used for testing alongside our old exchange server, but is also the hostname of the zimbra server.

I'd like to remove or rewrite both of these so any emails going outside the email server are from user@publicdomain.com

There are two issues sprouting up from having the ADdomain.local there.
The GAL's results are always user@insidedomain.local when a user is searched for. This is the lesser of the two issues, but I think ultimately it's causing the larger issue.

The Larger issue is that on occasion ZimbraUser1will email OutsideUser@separatedomain.com cc'ing ZimbraUser2. ZimbraUser1's email will be addressed FROM zimbrauser1@outsidedomain.com. The CC'd user's address will show up as zimbrauser2@ADdomain.local as it was selected from the GAL or Address Book, which causes an issue if OutsideUser wants to Reply all to the message as ADdomain.local is an invalid domain name for sending mail.

If possible I'd like to remove ADdomain.local but don't know if we can. Email should not be sent to that domain, and should only exist for authentication.

Additionally, nobody is using the zimbra.publicdomain.com, but i'm afraid to remove that from the domains as it's the host name of the server and again, don't know if it would break anything.

Please let me know what my options are, I appreciate the help in advance.