SMTP relayhost

[hede]

PS: Even if you are using the same port for both (i.e. port 25 for
incoming mails from your relayhost and local authorised clients) 
there should be a solution for your problem. Maybe using 
header_checks? [1]
Are there special headers for mail comming from the central ISPs smtp? 
Is it possible to add those? Then all incoming mail could be forwarded
to the ISP and mail comming back from it could be FILTERed to the local
transport (smtp-amavis oder local?).

Maybe there are even smarter solutions...

[1] http://www.postfix.org/header_checks.5.html