Spam Filters
Spam filters can be simple and local (just on your system), or elaborate and global (for an entire mail system).

At the simple end, almost all E-mail clients let you set up rule-based filters. For example, you could set up a filter that will discard, unopened, any E-mail that arrives with "vacation giveaway!" or "act now!" or "be your own boss!" (etc.) as the subject line. Other similar filters look inside the body text of the E-mail.

While it's time-consuming to create many filters, some spam--especially spam generated by multilevel marketing schemes--reuses the same or (nearly so) subject line at each level of the pyramid organization, making it relatively easy to weed out whole generations of unwanted mail with one filter.

But all simple filters suffer the drawback of being blunt instruments. For example, I sent out a recent newsletter that discussed network security for remote workers and other people who may conduct some or all of their office work at home. Alas, a number of body-text mail filters intercepted and blocked that newsletter because it contained the forbidden phrase "work at home," which often is associated with get-rich-quick spam schemes. Simple filters can't tell a legitimate use of certain keywords and phrases from a bad, spam-based use, and thus can end up tossing perfectly valid E-mails along with the spam.

Address-based filtering can avoid much of this kind of trouble, and is becoming increasingly popular. For example CAUCE recommends the free mailbox service from http://mailcircuit.com. (A similar service is available from http://chooseyourmail.com.) These services "white list" approved senders.

Here's how they work. You sign up with the service, which becomes your primary mailbox. When your mailbox receives a message, the software on the mail server checks to see if the arriving E-mail is from a known address on your "approved" list. If not, the service places the message in limbo and mails an auto-reply, asking the sender to reply via a coded URL or mailbox.

If the sender replies as requested, in effect proving his or her identity, then the original message is allowed into your mailbox and the originating address is added to the white list of approved senders. If the sender doesn't reply to the message (and most spammers either don't or won't reply) then the mail is discarded, and the originating address is placed on a black list that will prevent all mail from that address from reaching the mailbox in the future.

This is clever, but it has drawbacks. For example, spammers often forge mail headers, which could result in a mail filter adding an innocent third party's name to the black list.

And then there's the issue of how these free services pay their bills. For example, MailCircuit says it makes money by acting as a trusted "middleman between you and leading e-commerce companies. You agree to let us send you information in the categories you select and we protect your privacy by never sharing your address with anyone. These companies give us the information and we send it to you based on your category choices. No invasion of privacy, no flood of unwanted mail and no spam."

A cynic might say that filtered spam is still spam, but at least it's sent to you indirectly, so your E-mail address doesn't end up in spam databases.

A larger drawback is that, while these kinds of filtered mailboxes may work for individuals, they may be inappropriate or unworkable in an enterprise setting. There, locally hosted server-level tools are often a better choice. The page at http://www.cauce.org/about/resources.shtml offers a ton of additional resources, applicable for both for users and companies.

Some other services really stand out, too.