I want to migrate from my current mail server (old_server) for my domain mydomain.com. old_server setup is Postfix+LDAP+Cyrus.

Now I want to migrate my domain mail to Zimbra server (zimbra), but I am considering option to leave current mail server working in the first phase, and then to only have subset of email addresses to be forwarded to zimbra server. It seems that zimbra refers this in their documentation as 'edge MTA'.

Current config

 mydomain.com 
             MX: old_server
<---------- smtp send
----------> smtp receive

New config

 mydomain.com 
             MX: old_server                      zimbra
<------------------------------------------- smtp send
----------> smtp receive ---- forward ---->  smtp receive 

I need following:

1. old_server to receive mail for my domain as before, but for some of the email addresses I want them to be delivered to zimbra server. I should be able to determine which email addresses will be forwarded.
2. I would like to avoid possible false spam detections for mails from mydomain.com due to this setup.

Questions:

• How should I configure postfix on old_server to support this mail forwarding?
• To avoid false spam detection, can I have outgoing mail from mydomain.com to be sent by zimbra or should I use old_server?
• Is there anything extra I would need to do in order to avoid possibility of my outgoing mails being marked as spam on other servers?