Dual connection fast route problems

Hi,

I am using IP Filter: v4.1.13 on FreeBSD 6.2 and have an issue I can't 
seem to resolve with the help of google or the ipf documentation I have 
found.  Perhaps someone can assist.  I am no expert with IP Filter but I 
have an okay grasp of it and have been using it for quite some time 
successfully for basic firewalling functions.

If there is a better list to ask such questions on please point me to it.

I also might be going about this entirely the wrong way, but I don't 
know of any better way.

Basically what I am trying to achieve is dual Internet connection to a 
single internal NAT'ed email server for receiving mail in a redundant 
fashion.  However I can't get the return packets to go back out the 
interface which is the non-default route on the gateway.  The packets 
come in fine and on the way back out the gateway I catch them with IP 
Filters "to interface:ip" syntax and try to push them back out that way. 
  This technique is called fast route I think.  The rule catches the 
packets going back out, and changes the IP correctly to the one 
specified in the rule however it does not re-route the packet into the 
specified outgoing interface.

Here is my setup

* Hosts/Connections

Gateway:
FreeBSD 6.2 - IP Filter: v4.1.13
3 network interfaces of interest

- primary internet connection: ste4
static IP: 1.2.3.4

- secondary internet connection: tun0 (pppoe)
static IP: 5.6.7.8
Simple ppp setup via /etc/ppp/ppp.conf

- Internal LAN : ste3 (ethernet) 192.168.1.1

Then we have on the LAN:

Internal Linux host running SMTP with IPs 192.168.1.15 and 192.168.1.16


* Inbound NAT config
The primary Internet connections NAT is done by a router beyond ste4, it 
does translation of 1.2.3.4:25 -> 192.168.1.15:25

The secondary Internet connection uses ipnat configured on the gateway 
to redirect traffic
rdr tun0 5.6.7.8/32 port 25 -> 192.168.1.16 port 25 tcp

The default route on the gateway points out ste4 to the primary Internet 
connection.

Inbound SMTP connections to 1.2.3.4:25 work fine as it uses the default 
outbound route on the gateway.

For the secondary connection I have a rule in the ipf.rules like this to 
catch the returning server -> client packets as they exit the default 
outbound route interface of the gateway (ste4).

pass out log quick on ste4 to tun0:5.6.7.8 proto tcp from 192.168.1.16 
port = 25 to any

I know this rule is catching the packets and logging them, but its not 
changing the outbound interface to tun0.  The IP does change to 5.6.7.8 
as I can see them with tcpdump sailing out the ste4 interface.

I have successfully used the fast route syntax in other places in my 
ruleset to affect the route of outgoing connections so I figure I have 
the syntax right.

Appologies if this is bastardry of ipfilter, but it seems in theory that 
it should work.

Any help is appreciated.

Regards,
Ross