I am trying to set up postfix and devliver mail to virtual users What is the correct mailbox_command?

There is a howto that lists this:

mailbox_command = /usr/bin/maildrop

But a howto for qmail has this

|preline /usr/local/bin/maildrop -d "$RECIPIENT"

For postfix, do I also need to specify the recipient? I suppose that unless I do, it wouldn't be able to auth the virtual user. Otherwise I would have to do a rewrite of the username to username\@domain.com in postfix configs and that means another extra step for adding users. I'd much rather postfix just handed it off to maildrop. Unless there is a clean way to do this in postfix.

Rewriting however would allow having both virtual and nonvirtual users. Can anyone post their examples on how they dealt with these issues using postfix + maildrop + courier-imap?

Also, if user doesn't exist, what happens? maldrop sends a bounce?