Greetings,
I'm hoping this is an easy question as I believe I've already fought through the toughest parts already!

Say private lan network of 10.0.0.X, and public network of 68.10.0.1 through 68.10.0.10, with a dual-homed Linux box at 10.0.0.1 (eth0) and 68.10.0.1 (eth1).

I've added devices eth1:2 through eth1:10 as aliases for the remaining public IP's and have several servers on the private lan mapped to these public IP's:
For nat
-A PREROUTING -d 68.10.0.5 -p tcp -m multiport --dports 25,47,80,143,443,444,587,993 -j DNAT --to 10.0.0.20

For filter
-A FORWARD -p tcp -m multiport -d 10.0.0.20 --dports 25,47,80,143,443,444,587,993 -j ACCEPT

This works great and I have all 10 IP's mapping to their respective private LAN servers.

My question is mainly for mail servers since these often require reversed DNS resolution (which are registered under their respective 68.10.0.X addresses)- mapped private servers all appear to source as from 68.10.0.1 source address vs their mapped 68.10.0.X address.

What kind of rule do I need from each server to make them appear to resolve from a different public IP? Is there anything that might break from this as the incoming traffic is filtered only as above?

Any hints, tips or feedback greatly appreciated! Thank you in advance.

You should configure a POSTROUTING SNAT as well for the 1:1 NAT for the host(s).

example:

Thank you for your speedy reply! The POSTROUTING rule was the missing link.

As this box is also a gateway for LAN users, I already have a topmost rule for
-A POSTROUTING -o eth1 -j MASQUERADE

...so from your example, -A wouldn't work but -I to insert this into the chain seems to do the job:
-I POSTROUTING -s 10.0.0.20 -o eth1 -j SNAT --to 68.10.0.5

Is this a good way to go or is there a preferred method or re-ordering these rules?

Thanks again for the help.

nah, that should do it. When you run an iptables-save (service iptables save) it will place them with -A rules in the file that is loaded, but they will be in the correct order as is currently running.

That is what I did for one of mine. I had the same -j MASQ for "general" client usage and needed to configure 1:1NAT for an internal host so used the -I to create the rule above the -j MASQ.