Sending Emails by IP and through a firewall

For security reasons weve got a firewall (i.e. Sonic wall pro) sitting in front of two servers sending emails, and Im curious on how best to set this up. The key requirement I have is that each email sent must take on the identity of the ip address assocaited to the server that sent it in order to ensure that if one ip get blacklisted as spam then the other continues going? Now I maybe over analyzing this but I unsure on the best way to do this i.e.  
1.      Do you foresee any complications if I host the public ips on the firewall router and use NAT to relate the local ip addresses accordingly i.e. will emails take the identity of the local ip? Or will it take the identity of the outbound ip nat routed to in the firewall
2.      Or will I need to have the public IPs on the box itself? In order to ensure that emails are sent and identified as being sent from a particular email .

As you can imagine both scenarios are valid however example 1) will be the more secure by hiding the internal ips but Im unsure how emails are assocaited to ips when sent so i'm not sure on the best way to go about this?

So with this in mind I would really appreciate your opinions along with any tips / tricks you could offer?

I hope this makes sense but if not please let me know and Ill try to elaborate more.

Thanks in advance
P