I've been looking for a way to do this as well. If you find something that works before us, you let us know please! =)

I doubt you would get much help from the courier developers. They think there is no room for extremely useful backward compability with slightly older than new email clients. Instead they wish to force upon the world, the use of new esmtp clients which barely work correctly with esmtp. If you have many clients, you don't want to be going and telling them to switch from what they have used and loved for the past years to something new, buggy, and pathetic.

I have looked into pop-before-smtp, but as well couldn't hack a way to make it work with the inflexible design of Courier. Closest I came was something that monitored "maillog" (which was tasking), and attempt to rebuild that dat file and reload courier. This proved horrible, as some clients like to leave their email clients checking every 2 minutes, and with the smtp services restarting each time, with the smtp file being rebuilt, it was becoming unstable and useless.

It would be a wonderful thing to see courier build that functionality into itself... would be the best solution. Hell, even allow setting of 'which' user that it monitors for an ip to allow relay from. Of course these are all wonderful and logical thoughts of whimsy, and will _NEVER_ happen under the current management of courier. So you best bet is (if its not too late) move to Qmail, and only use courier as IMAP secondary support.