This category is specifically for server-level spam filters.
Spam filters that operate on individual mailboxes (that is, mail filters that operate at the client level) belong generally in Computers/Software/Internet/Clients.
A spam filter that is a plug-in to a specific mail-reading program (or is a description of how to write filtering rules for that program) belongs in the category for that mail reading program. (For example, spam filters for Eudora belong in Computers/Software/Internet/Clients/Mail/Eudora.)
This category is for anti-spam programs that operate on mail servers. The difference between an anti-spam program running on a mail server and an anti-spam program operating on a mail client is that the filter running on the server can remove spam from everyone's mailbox at the same time, instead of from just one user's mailbox. Server-level programs are installed by a systems administrator, and individual users do not need to maintain them.
Spam filters for a specific mail server (for example, writing filters for sendmail) are listed in the category devoted to that mail server. This category contains standalone software (and hardware) designed for spam filtering.