Allowing specific sender(s) to send otherwise banned attachmeent

Because of recent sprawl of /cannibalized spam/virus mail with
password-protected attachments/ we have (generally) banned ZIP files.


However, there are some legitimate senders who periodically (say, once a
month) send legitimate encrypted ZIP files to many of our users.


What would be the best / easiest way to allow these legitimate sender's
encrypted ZIP files to pass?


   Regards,

   Danilo