Zimbra Multi MTA Outbound

[rodale001]

Hi, and thanks for the time shared

I need to explain my environment
In out company we need to implement a mass mailing server
for this we have several public IP available and what we do is the following:
A multi-server Zimbra installation with the following

1 centralized storage and ldap for all accounts
2 several servers where you can send outgoing mail

I know its possible to install Zimbra on multiple servers and install several MTA servers to response but:

How can I do to send all my outbound email through one of all the public IP's taken them randomly and not to be mark as a spam server

Thanks