I looked around in the forum before I posted this and I found some pointers for Exchange, but I was wondering what are my options if I don't use Exchange?

Here is what I would consider to be usable solutions:

1. Existing Mail Server (Primary)/Zimbra (Secondary)
- Zimbra MX has lower priority than existing mail server
- Not every account will be in Zimbra
- Every account that is in Zimbra should have all mails existing on the mail server
- Sending emails must be possible between all accounts
- Only Zimbra accounts would have calendars/contacts
- Possible setup?

2. Zimbra is IMAP client to existing mail server
- Impact on calendar/contacts?

3. Existing mail server acts as a gateway domain server
- Forward mail by domain or SMTP
- Possible setup?

Basically, we don't want to disable our existing mail server while evaluating Zimbra.

Looking for some answers to the questions above or maybe some suggestions on how to do it otherwise.