By Doug Lowe

The most effective way to eliminate spam from your users’ inboxes is to use antispam software. Antispam software examines all incoming email with the intent of distinguishing between spam and legitimate email. Depending on how the software is configured, email identified as spam is deleted, moved to a separate location, or simply marked as possible spam by adding a tag to the email’s subject line.

Antispam software works by analyzing every piece of incoming email using sophisticated techniques that determine the likelihood that the email is, indeed, spam. When a certain threshold of probability is reached, the email is deemed to be spam and deleted, moved, or tagged. If the threshold is not reached, the email is passed on to the user as usual.

Microsoft Exchange mailboxes include a Junk folder that is often the ultimate destination of email identified as spam. You should always check your Junk folder whenever you can’t find an email you’re expecting.

Not all antispam programs use the Junk folder. Some programs store spam email outside of the user’s mailbox, in a separate location on the network or perhaps on the cloud. These programs usually deliver a daily email (often called a digest) that lists the emails that were identified as spam. You should review this email whenever you can’t find an email you’re expecting.

Determining whether an email is spam is not an exact science. As a result, false positives (in which a legitimate piece of email is mistakenly identified as spam) and false negatives (in which a spam email is not detected as spam and makes it into the user’s inbox) are not uncommon. False positives can result in your users not receiving emails they’re expecting. False negatives can leave users scratching their heads wondering how in the world the spam filter didn’t catch the spam. Sometimes email that to a human is obviously spam slips right by the antispam software.

The challenge of any antispam tool is finding the right balance of not too many false positives and not too many false negatives. Most antispam tools let you tune the filters to some degree, setting them to be more or less permissive — that is, erring on the side of more false negatives or more false positives. The stricter the filters are set, the more false positives you’ll have. Loosening the filters will result in more false negatives.

The possibility of false negatives is one of the main reasons that it’s rarely a good idea to configure an antispam program to simply delete spam. Most programs can be configured to delete only the most obvious spam emails — the ones that can be identified as spam with 100 percent certainty. Email that is probably spam but with less than 100 percent certainty should be marked as spam but not deleted.