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Spam filter is a software tool that is used to look through the incoming email messages and filter out any junk emails also known as SPAM.

A Spam filter can be independent software or a part of a package that provides email security.

Popular components where Spam filter is normally installed are Internet Mail Servers, private email servers, gateway level devices or even personal computers.

The reason we need to block spam is for the fact that not only are they annoying, but also are the main medium through which malware spreads.  Spam filter are welcome software that provides protection for computers as well as entire networks.

On the core, spam filter works by comparing the predefined set of rules configured in the spam filter engine. For instance, the subject of the email messages can be checked to see whether they contain any words or combination of words that might be linked with pornography or medicines or other common products used for spamming.

Other options like the sender’s domain can be checked against a set of predefined domains names or the IP can be checked to see if that source IP is known to be sending SPAM messages.

It is not that spam filters will be able to provide 100% spam filtering as the spammers have become intelligent as well over the years and have evolved and come up with new techniques that are used to bypass the spam filters.

For these reasons multilayered spam filtering approach is adopted across most of the organizations around the world. The ISPs provide spam filtering services to the subscribers who have opted for this service.

Further, the organizations would install the spam filters on the perimeter of their network and hence have the second layer of spam defense. And finally the user systems, either desktop or laptops, are also equipped with host level spam filter solutions making it a 3 tier spam filtering.

The common method of spam detection uses the Bayesian statistical methodology and according to this method, the system learns itself based on the emails that are received. The spam filter would separate the emails out in junk mail folder and inbox depending on whether they are spam or clean respectively.

The user can later review the mails in the junk mail folder and confirm if they are really spam emails. If not, they can mark those emails as safe and the spam filter would learn based on this action.

Spam filters also have the capability of allowing the users to build custom black and white lists based on email addresses, IP addresses, domains, etc. Users can specify which email addresses do the spam filter always need to trust and which IP addresses and domains are deemed safe.

The users can customize these lists as and when needed and they would take higher priority compared to the other filtering options that might have been set in the spam filter either by the user of the spam filter.