Postfix as a proxy to Exchange server

More and more people seem to be using an open source mail server on linux, such as Postfix, to proxy e-mails coming in from the net and relaying them to their exchange server. I know I’ve had this type of setup since January and it has been working really well for me. It gives you the ability to do advanced spam and antivirus filtering on messages, while keeping the easy to use GUI interface for creating exchange mailboxes. When will we get a great e-mail client so we can finally ditch the Exchange/Outlook setup that most businesses rely on? I know I haven’t found a solution that comes close (I’m sorry, Evolution for Win32 needs to come a bit further, and Thunderbird isn’t even close).

Anyway, once you have this system set up (there are some great instructions here, maybe I will cover this more another day), you may wish to sync up your Exchange users with your postfix “relay users” in order to trash messages coming in who are not addressed to anyone on the Exchange server. This will free up CPU cycles on the exchange server postfix server, and also reduce some bandwidth. Fortunately, Exchange 2000 and beyond use LDAP to publish this information. You can use Perl’s Net::LDAP module to grab this information. Chris Covington put together this nice script to grab the Exchange users and post to a file, which can then be postmapped and used in relay_recipient_maps. I hope you find it as useful as I did! [Local Mirror of the Script]