I'm sure that most, if not all, of you use some sort of mail-filtering software to help eliminate unwanted email. Some mail-filtering solutions are server-based, some are desktop-based, and some are a combination of both.
I use a desktop-based mail-filtering solution on my personal desktop system, and so far it works fairly well. As with many mail filters, mine has to be trained to recognize unwanted email messages and considers any messages that don't meet enough spam requirements to be legitimate messages. The good thing about this approach is that it decreases the possibility that I might not see a legitimate message that I really need.
The downside of the approach is that it takes a while to train the mail filter to properly filter as much spam as possible. As each message is processed, more keywords (typically called tokens) are added to the spam-filtering engine. So naturally the more spam the engine filters, the better it operates. I receive a lot of junk mail. For example, in August and September, I received over 28,000 email messages. Of those, at least 18,090 (more than 64 percent) were spam.
One thing I've found that really helps reduce the amount of spam that reaches my inbox is that my email filter supports the use of blacklist services. You might already know that blacklist services track IP addresses that are known to be used to send spam. So any mail filter that supports blacklist services can query the services for a given IP address (the sender's address or any address that might have relayed the message along the way). If the IP address is on a blacklist, then it's more probable that a message is spam.
In my testing of mail-filter software, I've found that a mail filter that uses blacklists should query every mail server found in a message's "Received:" header. Doing so increases the likelihood of detecting spam messages. But some mail filters don't query all the "Received:" headers, so they're less effective.
If your mail filter supports the use of blacklist services and you aren't using them, consider testing them to see if they help reduce the amount of unwanted email that you receive on your network. Blacklist services are somewhat controversial because of complaints that some services blacklist IP addresses at the drop of a hat without much, if any, investigation first. In my experience thus far, services such as SpamCop, Spamhaus, Relay Stop List, and Spam and Open Relay Blocking System (SORBS) work fairly well. To find other possible blacklist services, use your favorite search engine to query for "blacklist services."