How does an E-Mail Filter Program Work?
Any letter, fax or e-mail that you get from a particular individual will be littered with similar words and phrases. The ones you send will similarly use the same words you've always used. It's just the way we are!
E-mail filters use this fact to their advantage in that they identify who sent an e-mail not just by the e-mail address but by the words it contains. They also use all the hidden information in the e-mail, such as its routing, who actually sent it, the return address and any links it contains to help in the identification. Many filters use what are known as Bayesian statistics to identify each message.
But the main thing about these filters, is that as they don't rely on the e-mail address, spam which always comes from new e-mail addresses, is generally recognised and labelled as such. So after a quick glance at the subject, you usually dump it directly.
Filters Can Catagorise Wanted E-Mails Too!
Using an e-mail filter can also be a great advantage with those e-mails that are important.
Now just as a filter can identify spam, it can also identify whether the e-mail is from an important client, a family member or your boss. So by matching a few parameters in your e-mail program to the filter, all of your messages can be delivered to an appropriate folder.
Mail Server or Desktop Filtering
The filter is generally placed in one of two positions; on the mail server or on the desktop.
Doing the filtering centrally is easier, but it does mean that sometimes your important messages may not get through. On the other hand, doing it on the desktop is very flexible and means that no messages get lost. Sometimes a combination of both filters is best.
Two filters; POPFile for the desktop and SpamStomp for the mail server are described in detail.
Helping Filters to Recognise You
You should also expect that when you send an e-mail, that the recipient may use a filter.
So always return their e-mail with a reply and not a new e-mail, as the internal information, e-mail address and any signature will be instantly recognisable to their filter and will be let through immediately.
Also always use a sensible signature on your e-mails. Recipients filters will learn that it is you and let the e-mail through unhindered.