If the router is doing PAT, then you have to set up a port address translation using port 25, and direct it to the PIX outside interface address. Then on the PIX, set up antoerh port address translation for port 25 and direct it to the internal SMTP server. On the PIX it will look like this (assuming the SMTP server's actual address is 10.1.1.1):
static (inside,outside) tcp interface 25 10.1.1.1 25 netmask 255.255.255.255
access-list 100 permit tcp any host 10.1.1.1 eq 25
access-group 100 in interface outside
or if you're using conduits, replace the last two lines above with:
conduit permit tcp host 10.1.1.1 eq 25 any
Timeouts, etc should be fine. I just set this up for a customer and it worked fine, just make sure both the router and the PIX are redirecting TCP port 25 internally to the right spot (the router to the PIX, then the PIX to the server).
