Foad aids in filtering spam. Often spammers use Bcc, create rogue
lists, or use rogue envelopes to send their spam. By keeping a list
of your valid email addresses and mailing list subscriptions (which
most folks do anyway), you could cut the spam that appears in your
in-box by half or move.
These functions could be done within procmail itself, but I find this
method much easier to manage and perhaps a bit more flexible. Foad is
small and fast, which also makes it a good candidate to be run before
heavy-weight anti-spam and virus scanning solutions like spam assasin.
See the INSTALL file in the distribution for installation instructions
and the manpage for configuration information and samples.
The current version is 0.3 and can be downloaded here.