First, I'm not familiar with Courier, but most IMAP servers also have POP servers. There are legitimate reasons you might not want to go that route (assuming that Courier has that feature), but I just wanted to make sure it occurred to you.

Second, I'm also not familiar with maildrop, but Postfix has a filter feature that might work for you. Read README_FILES/FILTER_README in the Postfix source distribution.