Razor is used to check message signatures against Vipul's Razor collaborative filtering network. Razor is not available from CPAN – you have to download it from http://razor.sourceforge.net/ .
Razor has a large number of dependencies on CPAN modules. Feel free to skip installing it, if this makes you nervous; SpamAssassin will still work well without it.
Note that Razor is not available for unlimited free use, so it is commented out in init.pre. It is currently free for personal use, subject to capacity constraints. See the Cloudmark SpamNet Service Policy for more details.
Note that Razor support does not seem to work on Windows systems. Win32 users should disable the Razor tests using "score RAZOR2_CHECK 0".
You (and any users who use Razor) will need to register an account to do so, like this:
razor-client [creates symbolic links] razor-admin -create [creates the razor home] razor-admin -register [registers an account]
To use Razor with SpamAssassin, you will need to be using version 2.40 of the Razor modules, or higher. v2.61 or higher is recommended.
More info on using Razor can be found at UsingRazor.