Just checking around my Zimbra system to make sure that everything is working OK, and I found this:-
/var/spool/postfix/maildrop contains quite a lot of files. These go back to when the system was first commissioned.
The /opt/zimbra/postfix/maildrop is empty, and I'm guessing this is where those files should have been put.
Should the /var/spool/postfix/maildrop be a link to the zimbra one?