Postfix delivery over IPv6, than ISP mail server

[Joris Dobbelsteen]

I'm using Postfix 2.3 (with debian etch, but I'm planning to go to 
postfix 2.5 with debian lenny). I'm using the postfix box solely for 
relaying e-mail and doing virus/spam scanning. Mail for local domains is 
relayed to a local server and everything else goes to my ISPs server. I 
currently use transport_maps to manage this and that is working fine for 
a couple years.

Now I want to reduce the use of my ISP mail server. For IPv4 this is not 
possible: its troublesome with residential access & my ISP blocks 
outgoing port 25. However I have an IPv6 ip address where mail delivery 
is available.

Is there any possibility to use the "transport_maps" to indicate 
multiple destinations?
I've seen reports that it didn't work for older versions. A few creative 
solutions where provided. Maybe someone else has a good idea?

(To be honest, it has been over 2 years ago that I have looked very 
deeply in how Postfix moves mail messages around all classes and I have, 
not suprisingly, forgotten the details in the meanwhile, because the 
configuration simply worked and did not need modification).

- Joris