[Exim] trying to "hold" messages with unknown local-part

    I am using a standard Debian 2.2 exim - only changes are a lot of local domains I added and a few /etc/aliases.
    I would like to do something like this: if someone sends mail to  john@???  and user "john" does not
    exist, I would like to "freeze" the mail in my queue - so that I choose to create the user or discard the message.
    Any ideas will be appreciated!!!