Security/Firewall question

[Gregg <gbtech () citlink net>]

Hi everyone!

I'm still pretty new to security and firewalls and such, and I'm having a 
problem wrapping my head around a couple of concepts. Here's what I have-
I have a stand alone email server behind an Adsl router (with 4prt hub). 
The router is set to pass-thru (nat and firewall disabled). 1 port goes to 
a firewall device, and my LAN behind that. 1 port goes to my Email server, 
a Win2k box (hey, quit lookin at me like that). 

I've got a handful of fixed IP's to work with. Here's what I'd like to do-

Keep everything the same BUT- put an OpenBSD box in between the router and 
the email server (protect the snivelling email server). So, I builts me 
dis purty OpenBSD box from the broken bodies of mine enemies past (a Dell 
Dim XPS V350 with a bad video card). Put 2 Nics in the beast. Lovely.

Now, I have an IP from my block of 5 registered currently for my email 
server. 
I'm not certain if- 
I want to assign that IP to the OpenBSD firewall, and use NAT and/or RDR 
to pass on SMTP traffic on port 25 to the email server. Yes? No? Maybe? Am 
I a shame on my species? 

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