Personal proxy

You can run a small proxy application on your client computer that will connect to your mail server, get the email, run SpamAssassin on it, then deliver to your email client.

That's a personal proxy server.

• JSpamAssassin is a POP3 Proxy Server developed in Java (platform independent). It has a friendly GUI and acts as a communication interface between the user POP3 server and Apache SpamAssassin. It runs on all OS and works with all e-mail clients (Eudora, Pegasus Mail, Netscape, Thunderbird, Outlook Express, etc.).
• ImapAssassin is a perl application which uses SpamAssassin to pre-filter an IMAP mailbox for spam, before you download it.
• No Spam Today! POP3 Proxy for Workstations (Windows™ 98SE/ME/2000/NT/XP/2003) easily connects to any POP3 email client, and provides a wizard to enable spam protection for all your Outlook™ POP3 mail accounts. It's $30.
• Pop3Proxy and SaProxy : Windows (free open source) proxy based on Dan Mc Donald original work. Setting up Pop3Proxy, and some versions of SaProxy, is *not* point-and-click easy, however. Altought there was one SaProxy version point-and-click easy, it is outdated (it includes an old version of SA)
• SaProxy Pro : by Bloomba, derived from SaProxy; Windows proxy that's easy to set up and works quite well. It's $30. Seem to be some claims regarding support
  
  

Site-wide proxy

You can also set up a spam proxy server that receive all your organization's incoming mail, filters it, and pass it to your organization's legacy mail server.

• No Spam Today! SMTP Proxy - Seamless SpamAssassin Integration Into Any Existing E-Mail Environment - No Spam Today! requires just a few mouse-clicks to integrate SpamAssassin into any existing Email infrastructure (supports Windows™ NT/2000/XP/2003 and Linux).
• any MTA configured as a satellite system and running SpamAssassin will do that (see IntegratedInMta). However, setting up an MTA requires careful configuration.
• The Amavis mail scanner can be set as a SMTP to SMTP proxy. Amavis sits between two SMTP mail relays, receives incoming mail, filters it through SpamAssassin or an optional virus filter, and drops, bounces or marks spam messages. Though you can't use personnal settings, it's very flexible and does not require you to mess with your MTA's configuration.
• MailScanner can be used in a similar way to Amavis.
• http://www.worlddesign.com/index.cfm/rd/mta/spampd.htm SpamPD can be used as an SMTP or LMTP proxy server. Other than amavis, it uses the original SpamAssassin style header/subject tags. It can't do anti-virus scans though.