Anti spam solutions, four simple model
1. Software model: Several software-based anti-spam solutions are available that you load on a dedicated server or right on your e-mail server.
2. Appliance model: In anti-spam solutions, the appliance acts as sort of an e-mail firewall, in that it logically is placed between the Internet and enterprise mail server(s). The anti-spam appliance examines every incoming mail message and, using a list of filtering rules, makes a pass or block decision for each message. I’ll grant you that software and ASP solutions also are e-mail firewalls — it’s just that an appliance solution also looks like one.
3. ASP model: ASP stands for Application Service Provider, meaning the application resides on a computer located elsewhere, and what you’re buying is essentially a data service, in this case e-mail filtering. Antispam companies offering the ASP model perform all the spam filtering on their physical (or logical) premises, and deliver only the clean e-mail to you.
4. Client-only model: Anti-spam software that is wholly contained on the end-user workstation is definitely worth considering if you have a small number of users (exactly how small is up to you). That’s because you could install it on workstations one at a time as users complain about spam, and you can control how that spam is filtered based on individual needs. However, this solution has its downside, too. Most client-side solutions offer no centralized management or reporting capability, and these solutions don’t keep the spam off your mail servers, because they don’t filter messages until they reach the desktop.[Spyware for Dummies, Peter Gregory 2005]