Nessus DOS attack against Sophie

We use Sophie from mimedefang and are verify satisfied with
it. However, one of my collegaues ran the Nessus security scanner
enabling 'dangerous' tests. As part of the test, it sent a
mail through our mailserver. It caused severe overloading of 
the filter machine. A zip file which was part of the mail contained zip
files containing zip files which untimately ended up in some verfy large
.dll files. I've searched the web for info about how to configuere
Sophos/Sophos to cope with such DOS attacks but have found no
solution. I'll be glad to hear if there's a solution our there so that
we are prepared the day somebody decides to launch a real attack
against us.

   -- Michael Glad, email: [email protected]
      CS Dept, Aarhus University Denmark.

NB: the zip file is available at http://www.daimi.au.dk/~glad/sophos-dos.zip. 
    Please don't blame me if it chokes your dog :-)