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LanZing.Net
spam prevention system utilizes
SpamAssassin.
which have several
custom written filters. There are several options such as the ability to
just mark spam or bounce it, whitelist and blacklist
users, change your spam threshold etc. If you choose to
bounce spam, we have a verification system where
customers can verify themselves and whitelist themselves
to allow all future mail through the system.
SpamAssassin is generally regarded
as one of the most effective spam filters, especially
when used in combination with spam databases. Even
simple text-matching alone may, for most users, be
sufficient to correctly classify a majority of incoming
mail. SpamAssassin comes with a large set of rules which
are applied to determine whether an email is spam or
not. To decide, specific fields within the email header
and the email body are typically searched for certain
regular expressions, and if these expressions match, the
email is assigned a certain score, depending on the
test, and several (customizable) headers are added to
the mail. The total score resulting from all tests or
other criteria can then be used by the end user or by
the ISP to set the conditions under which email is moved
to a separate spam folder, deleted, flagged etc.
Each test has a label and a description. The label is
usually an all upper case identifier separated with
underscores, such as "LIMITED_TIME_ONLY", with the
description for that label being "Offers a limited time
offer". A mail that passes that test (in this case,
contains certain variants of the "limited time only"
phrase) might be assigned a score of +0.3. With a spam
threshold of 5 (default as of version 2.55), several
other tests would usually have to pass for the mail to
be classified as spam. On the other hand, some tests,
such as those for invalid message IDs or years, result
in a very high score being assigned, where even a single
test can almost put a mail "over the edge".
When a mail's total score is higher than the "required_hits"
setting in SpamAssassin's configuration, the mail is
treated as spam and rewritten according to several
options. In the default configuration, the content of
the mail is appended as a MIME attachment, with a brief
excerpt in the message body, and a description of the
tests which resulted in the mail being classified as
spam. If the score is lower than the defined settings,
by default the information about the passed tests and
total score is still added to the email headers and can
be used in post-processing for less severe actions, such
as tagging the mail as suspicious.
The user can customize these filters using a file "user_prefs"
in their home directory. Within this file, they can
specify individuals whose emails are never considered
spam, or change the scores for certain rules. The user
can also define a list of languages which they want to
receive mail in, and SpamAssassin then assigns a higher
score to all mails that appear to be written in another
language. This can be very useful to users receiving a
lot of foreign spam but never actually corresponding
with people in that language.
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