A common tool people use for managing spam is SpamAssassin, and if you are using it, or just interested in learning more, you’ll want to check out my new Guide to Writing your own Add-On Rules for SpamAssassin.
Among the many helpful tips it offers, there’s an entire section on why you might or might not need custom rules for SpamAssassin, which states:
“Since custom rules are generally not needed, why would you want to use some? Well, the default SpamAssassin ruleset is tuned to a somewhat generic sampling of mail that comes from the corpus submitters. If the typical kinds of email coming to your network are significantly different than those used in the corpus, you may get a lot of false positives or false negatives. Whitelists, blacklists and tweaking scores of the default rules can help, but might not be sufficient in all cases.”
Lots of useful information.