Is it possible (or even logical) to use Postfix as relay MTA with Microsoft Exchange Server 2007, and if so how?
2 Questions here.. Is is it possible, yes in a backup MX kind of way.
I use Postfix as the backup MX SMTP server, so when the main SMTP point is down on the exchange server (due to reboot, patching or whatever) the linux box will store any overflow mail that the exchange server couldn't cope with.
How I do it is with MX records in the domain files, specifically following this page will set you on the right path. It's actually pretty simple from the postfix side, just remember the DNS side, the lower the MX number, the higher the priority.
When I think of people wanting to use Postfix with Exchange, it is usually in terms of using it as a gateway so that it sits between Exchange on your internal network and the internet. With this configuration, you point MX to only the Postfix box and it forwards all inbound mail to Exchange. Similarly, Exchange forwards outbound mail to Postfix which then handles internet mail delivery. The advantage here is that you can use Postfix for email hygiene (antispam, antivirus) and keep your Exchange server off the internet. Of course, it's true that Postfix can't forward directly to the Exchange mail store but it depends on what you mean - it can't totally replace Exchange's MTA but it can certainly forward to and from Exchange via SMTP. Since you're running Exchange 2007, think of Postfix as a replacement for the Exchange Edge Transport role.