I am running a mail server with virtual users through a userdb database. As I have neither of the mysql, postgres or ldap USE flags, maildrop is built with --disable-authlib, therefore disabling the virtual user functionality. Currently, I corrected the problem by adding the "mysql" USE flag for maildrop in packages.use. However, it might be a good idea to add a separate "authlib" USE flag for this case.
Fixed in mail-filter/maildrop-2.0.2. Thanks