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
four 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, it moves them to a separate IMAP or
Exchange mailbox. 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 generally does not adapt to your
own mail.
- Client-Side Integrated
- This category includes SpamSieve and Apple Mail’s built-in spam filter.
Suspected spam messages are moved to a separate mailbox, 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.