NAI WebShield SMTP does not scan base64 encoding

[chris.paget () ANALYSYS COM (chris.paget () ANALYSYS COM)]

While investigating todays virus outbreak (Stages.Worm), I noticed
that our email virus scanner (NAI WebShield SMTP 4.5, engine 4.0.50,
DAT 4.0.4082, 14/06/00) was not picking up all attachments.  
The server is configured to block all SHS, VBS, etc attachments, and
notify the sender.  However, when these are sent as Base64 encoding
(rather than 8-bit), they are passed by the server, and could
potentially infect the network.  8-bit attachments are successfully
scanned (and blocked if necessary).

Chirs