...
auth_mechanisms = plain
passdb {
    driver = passwd-file
    args = /etc/dovecot/passwd
}
userdb {
    driver = static
    args = uid=vmail gid=vmail home=/home/vmail/%d/%n allow_all_users=yes
}
service auth {
    unix_listener auth-client {
        path = /var/spool/postfix/private/auth
        mode = 0660
        user = postfix
        group = postfix
    }
    user = root
}