Filtering is necessary to prevent spam from ever reaching your inbox. If you already have a compromised email address and cannot, or do not want to, change it, filters are the only way that you will be able to effectively manage your spam problem. There are multiple ways to accomplish spam filtering, each with varying levels of success. The most common method is to use a signature-based filter.
Signature-based filters compare incoming email with email known to contain spam. The various companies that offer this service collect spam using fake email addresses known as honey-pots. Once they have successfully lured a piece of spam, they create a signature for that email. This is done by assigning a unique value for each character and then totaling the values to calculate a result. This becomes the spam signature. The signature is then added to the company's database and sent as an update to the subscriber's email.
These filters are very good at preventing false positives. The signature assignation process makes it highly unlikely that any other piece of email would have the same signature as the spam it was given to. This keeps your letter from Grandma from getting deleted as possible spam, a problem better filters often have. What good is a filter if you can't receive the mail that you want?
The downside to these filters is in total performance. Unfortunately, spammers employ techniques to circumvent signature-based filters. The easiest way for them to do this is to use a program that generates random characters in each email. This will create a separate signature for the spam in the database and the spam you are receiving, even though it is essentially the same thing. The newest generation of signature filters attempts to combat this randomization by specifically looking for it in the spam. The problem that exists with these spam filters is the same for virus protection services. They assign a unique value to a known virus, but the authors and distributors of the virus continue to change it slightly creating a need for new signatures for the same thing. Sometimes, they change randomly, or mutate, as part of the program. Spammers are just as malicious and have money to make. There is nothing preventing them from using these same tactics.
Additionally, signature-based filters are tracking spam that has already been sent. By the time they have been assigned a signature and updated to your email service, the spammer has already sent millions of pieces of spam. Nothing prevents you from receiving the same spam as the honey-pot within that time. This is again is very similar to anti-virus programs. They can only update based on what they currently know about. Spammers know this and simply change the next batch of spam once it has been given a signature. Any good spammer will maintain dummy email accounts that they use spam filters on and then test the spam by trying to get it through their own filters. They have a lot of time and resources available to them to make sure most of their spam is getting through.
For these reasons, signature based filters are only about 65% effective at blocking spam. To be worth the effort, and possible loss of good email, a spam filter needs to be above 90% effective. They provide a good first line defense when used in conjunction in other filters, or if spam is not too large of a problem for you. Understand that when spam does become more of an issue to your inbox, you will probably need to switch to another type of filter.
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