That's fine, if you have two public IP's available and one being utilized by your PIX outside interface then you can use the other IP (assuming that the 2nd IP is not for your internet facing router) for your mail, you should have the following (below) on your PIX config. Now all traffic that goes out to the internet from you LAN will be NATd to one IP address (i.e. your PIX outside IP address).
ip address outside 255.255.255.x
ip address inside 255.255.255.0
global (outside) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0 0 0
route outside 0.0.0.0 0.0.0.0 1
If you now need to allow mail access to an internal mail server, youll require ACL and static translation applying so to map the outside mail traffic to your internal mail server. You can use your other available public IP to achieve this but make sure that your MX record for mail points to this IP.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...