WebCentral
WebCentral's SpamTrap is an easy-to-use POP 3 internet e-mail filtering service that sits before your e-mail server. Spam can be viewed online 30 days after it has been sent.
www.webcentral.com
Clearswift SpamActive
Managed anti-spam filter freely available to users of Clearswift products which pushes automatic updates out to your server, allowing you to choose if you want to receive your spam or have it looked after by Clearswift.
www.clearswift.com
MessageLabs
MessageLabs' fully managed offering sees spam scanned at their location using a global database of known security threats and spam. Operating at Internet level, MessageLabs offers a level of control before the e-mail comes through your network boundary.
www.messagelabs.com
MailGuard
MailGuard offers free set-up for anti-spam, with a monthly cost following which depends on the number of e-mail users you have. This covers anti-spam, anti-virus, content filtering, and other services.
www.mailguard.com.au
Trend Micro
A hosted anti-spam offering provided with Trend Micro's inegrated gateway anti-virus suite free of charge.
www.trendmicro.com
IC Consulting
A hosted anti-spam offering that comes with IC Consulting hosted services that uses open source software.
www.icconsulting.com.au
Telstra
Filters e-mail from your BigPond inbox into a separate spam folder that is automatically created in your telstra.com WebMail account.
www.telstra.com
This article was first published in Technology & Business magazine.
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I think hosted anti-spam IS the answer. I use the Gmail solution: http://www.iopus.com/guides/gmail-spam-filter.htm