Spam and Anti-Virus

[Chris Purcell]

We're currently running our mail gateway on a Windows server running GFI
Mail Security and GFI Mail Essentials.  We desperately want to move our
gateway to a Linux server.  What spam and anti-virus software would you
all recommend for a mail gateway?  It can be open source or commercial,
we're willing to pay for a high quality product.  We have about 500 users
and currently get about 100,000 pieces of spam per month.  The spam
software must do bayesian filtering and it would be nice if each user
could train its own mail and release any quarantined spam without the help
of an administrator.

Thanks,
Chris