hi

I'd like to setup following:

Internet -> Postfix -> Exchange

Postfix gets ALL mails from Internet and sends to MS Exchange 5.5,
but Exchange sends ALL mails directly to the Internet (without postfix).

regards
cccc

install postfix and read /path/to/postfix-config/transport or man transport. It's easier than you think I'd bet.

Take a look at the postfix transport table. i.e. /etc/postfix/transport Thats what I do to relay all e-mails for my domains to my exchange server.

But be warned... Implementing this feature without first configuring postfix to check if the mailbox exists on exchange can lead to disaterous results. i.e. A spammer doing a dictionary attack on e-mail addresses within your domain. Postfix will relay the inbound e-mail to exchange. Exchange will then generate a non-deliverable DSN stating mailbox not found and start filing up its queues.

So before throwing the transport table switch on your postfix server, checkout the "relay_recipient_maps" option in /etc/postfix/main/cf. Implementing this feature in postfix causes postfix to first check if the mailbox exists on exchange before relaying. FWIW: I use LDAP queries against my exchange server to test the mailbox, but you can also create a static file of valid e-mail addresses.