[Exim] Frozen messages with null envelope sender

Hi

Is there a way to drop messages instead to freeze them, if (and only if)
they have null envelope senders? Such as:

25d   32K 1AqqCD-0004c2-BN <> *** frozen ***
          dan@???

I have lots of these mails, as everyone will, because of virii using forged
addresses.

Thanks,
Steffen