Has anyone migrated from postfix to Exchange and moved saved mails forward? I have been asked to move about 150 mailboxes, and I would rather avoid going down the Outlook/PST route if possible.
Am sure on the postfix system you are running some sort of imap server, you can use imapsync to migrate the mail across to the exchange server while retaining the message flags.
What version of Exchange? If it is small business server, you could use the POP3 connector to pull mail from the mailboxes. (Postfix by itself doesn't host mailboxes, you must be using something else alongside postfix to do this).
Obviously, I don't know your set up, but I'd recommend keeping postfix, as it integrates quite nicely with Exchange/Active Directory. Our company uses this set up, as postfix performs our virus and spam filtering (postfix, spamassassin, clamav, amavis), plus it means I don't have to expose port 25 of my exchange server to the world (which makes me very nervous!).
Edit: Just saw you have 150 mailboxes, so you won't be running SBS. An alternative would be to use fetchmail to do the job.