Helllo guys, this was my "before" topology:

Internet----CISCO 1750-----Cisco Catalyst (VLANS)------HUB------Mail 
Server(public)
                                                      
And this is the "after" topology:

Internet----CISCO 1750-----Cisco Catalyst 
((VLANS)-------m0n0wall-------Mail Server(private)

As you can see, now I have a private IP for my mail server, I have a 1:1 
NAT ruel, so I had to define a proxy ARP in the WAN interface of m0n0 in 
order to work.
But...this isnt working!...UNLESS I spoof the MAC of the mail server in 
the WAN interface of m0n0wall.
The Cisco, doesnt have a static ARP table or nothing similar...so I'm 
loss.....I hope you can figure out or comment some common problem that 
maybe I forgot to check.

Thanks a lot.