Morten Winther wrote:

 > Just imagine a hotel with guests trying to send mail - all of them
 > using there 'home net ' smtp server which should result in a relay
 > access denied.

I use my laptop at different places, and also occasionally dialin
to my provider. Yet, I don't really often have to reconfigure
because I have only "smtp" as the name of the outgoing mail server.
This is always relative to the domain I get assigned through DHCP,
and in many cases ends up at exactly the right server.

So I think the most reasonable you can do is:
- make sure your mail relay is reachable as "smtp.your.domain"
- allow smtp.your.domain:25 tcp
- deny all other outgoing smtp

But don't simply hijack smtp traffic. That would be rather impolite.

--B