by Kirill Popov and Loren McDonald

Got filtered? Of course you have. Your messages have been filtered by ISPs for the past few years.

E-mail filtering is a necessary part of fighting spam. As ClickZ News reported earlier this year, AOL receives roughly 2 billion e-mail messages a day, of which about 75 percent are blocked and another 4 to 7 percent are sent to the bulk folder.

Dealing with this influx of spam is a major issue for ISPs. It's a tremendous drain on servers and human resources. It's also a major source of customer complaints, dissatisfaction, and churn. Minimizing the flow of spam to inboxes has become a major point of differentiation as ISPs battle to acquire and retain customers.

Long ago, smart marketers moved past the anger phase, all the way to acceptance. They're actively deploying a number of techniques to minimize ISP filtering.

Like the search engines' secretive page-ranking algorithms, ISPs don't share the specific types of filtering they deploy nor at what thresholds. Through dealing with ISPs over time, it's apparent there are quite a few key components. Though they may be approached differently at various companies, all are quite commonly used.

ISPs deploy a number of methods to reduce the volume of spam.  Learn about them in the full article on ClickZ: Understand ISP-Level E-Mail Filtering.