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1.3   Integration

Separate from the issue of identifying spam messages is the issue of how to prevent you from having to deal with them. There are basically six kinds of anti-spam software for doing this:

Challenge-Response Systems
This software requires people who send you mail to prove that they are human, and not an automated spam-sending program. After sending you a message, they get a reply asking them to complete a task that is easy for humans but hard for computers. Only then is the message passed on to you. This system is a nuisance for senders, delays your reception of the mail, and becomes impractical when sending messages to a group of people. Also, challenge response systems cannot deal with spoofed senders or legitimate messages that are sent by programs.
Server-Side Filters
This software runs on mail servers and often filters out spam before you ever see it. This means that you do not have to download the spam messages that it catches. However, some spam messages may still get through, and, unless the filter is perfect, a few legitimate messages will not. These could be important messages, and you will never know that you lost them.
Server-Side Taggers
This variant of server-side filters does not delete possible spam messages before you download them. Instead, you download every message and configure your e-mail program to move messages that were tagged by the filter into a separate spam folder. This eliminates the major disadvantage of server-side filters—lost messages—however this type of filter is generally not as accurate as the ones below, because it does not adapt to your own mail.
Client-Side Filters
This software connects to your mail server to delete spam messages before your e-mail program can download them. This is a clunky approach: to catch all the spam messages, you have to run the program right before your regular e-mail program checks for mail. This is difficult to time properly if you check your mail often, and even so you may download some messages that weren’t filtered. You will also download every good message twice. The anti-spam software may let you see the messages that it filtered out, so that you can verify that there were no false positives. However, you have to do this using its interface, not your e-mail program’s (which is typically nicer). And if there was a false positive you then have to transfer it into your e-mail program so that you can file and reply to it.
Client-Side Proxies
This is like a client-side filter except that the proxy downloads messages once and stores them locally. The e-mail program then “downloads” the good messages from the proxy. This addresses the timing and double-download problems of client-side filters, but interaction with the filter is still awkward because it happens outside your e-mail program. In addition, you lose some control over the connections to the mail server and which messages are left on the server.
Client-Side Integrated
This category includes SpamSieve and Apple Mail’s built-in spam filter. Suspected spam messages are moved to a separate folder, which you can quickly scan at your leisure to make sure there are no false positives. The e-mail program downloads messages directly from the mail server, thus avoiding the problems of client-side filters and proxies. You can train the anti-spam software to improve its accuracy from inside your e-mail program, and accuracy is higher than with server-side filters because the anti-spam software can learn from the messages that you receive. You can also control how the spam filter interacts with your regular mail sorting rules.
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