Mail Service Filtering
Mail service uses SpamAssassin (spamassassin.apache.org) to filter 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 configuring the junk mail and spam filters follows in these sections:
ÂÂ “Enabling Junk Mail Screening (Bayesian Filters)” on page 35 ÂÂ “Training the Junk Mail Filter” on page 36
ÂÂ “Filtering Mail by Language and Locale” on page 37
ÂÂ “Enabling Virus Screening” on page 38
Enabling Junk Mail Screening (Bayesian Filters)
Before you can benefit from mail screening, it must be enabled. While enabling screening, you configure screening parameters.
Bayesian mail filtering is the classification 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
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 filter assigns the incoming message a probability of being junk or good, depending on what group it most resembles.
Bayesian filtering has shown itself to be a very effective method of finding junk mail if the filter 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 classification is. Even if junk mail senders alter their mailings, the filter takes that into account the next time around.
To enable junk mail screening:
1In Server Admin, select a computer in the Servers list, then select Mail.
2Click Settings.
3Select the Filters tab.
4Select Scan Mail for Junk Mail.
5Set the level of permissiveness (Cautious, Moderate, Aggressive).
The permissiveness meter sets how many junk mail flags 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.
Chapter 2 Mail Service Setup
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