The version of maildrop bundled with Courier behaves differently than the standalone version - the account information is read from environment variables. I don't know what you did, but you should not even be seeing maildrop reading userdb.dat at all, Courier would do that, and provide the required information (home directory, default mailbox, etc...) via environment variables. Either you've messed up and installed the standalone build of maildrop with Courier, or Courier's authuserdb module is not being used.