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Chapter 2 Mail Service Setup 35
Mail Service Filtering
Mail service uses SpamAssassin (spamassassin.apache.org) to lter spam, or junk mail,
from incoming mail messages. Mail service uses ClamAV (www.clamav.net) to detect
viruses in mail messages. Both tools are managed within the Filters pane of Mail
Settings in Server Admin. Additional information on conguring the junk mail and
spam lters follows in these sections:
“ Â Enabling Junk Mail Screening (Bayesian Filters)” on page 35
“ Â Training the Junk Mail Filter” on page 36
“ Â Filtering Mail by Language and Locale” on page 37
“ Â Enabling Virus Screening” on page 38
Enabling Junk Mail Screening (Bayesian Filters)
Before you can benet from mail screening, it must be enabled. While enabling
screening, you congure screening parameters.
Bayesian mail ltering is the classication of mail messages based on statistics. Each
message is analyzed and word frequency statistics are saved. Mail messages that have
more of the same words as those in junk mail receive a higher marking of probability
that they are also junk mail. When the message is screened, the server adds a header
(”X-Spam-Level”) with the junk mail probability score.
For example, let’s say you have 400 mail messages where 200 of them are junk mail
and 200 are good mail. When a message arrives, its text is compared to the 200
junk mail and the 200 good messages. The lter assigns the incoming message a
probability of being junk or good, depending on what group it most resembles.
Bayesian ltering has shown itself to be a very eective method of nding junk mail if
the lter has enough data to compare. One strength of this method is the more mail
you get and classify (a process called training), the more accurate the next round of
classication is. Even if junk mail senders alter their mailings, the lter takes that into
account the next time around.
To enable junk mail screening:
1 In Server Admin, select a computer in the Servers list, then select Mail.
2 Click Settings.
3 Select the Filters tab.
4 Select Scan Mail for Junk Mail.
5 Set the level of permissiveness (Cautious, Moderate, Aggressive).
The permissiveness meter sets how many junk mail ags can be applied to a message
before it is processed as junk mail. If you set it to “Least permissive,” mildly suspicious
mail is tagged and processed as junk mail. If you set it to “Most permissive,” it takes a
high score (in other words, many junk mail characteristics) to mark it as junk.