How Spam Filtering Works
Spam filtering is a two-step process: scanning and marking incoming messages, and delivering messages marked as spam to a different mail folder.
Scanning for spam
All mail coming in to Dartmouth is scanned by a piece of
spam-recognition software called "SpamAssassin"; you don't need to do
anything to have this happen. Note that SpamAssassin itself does not
filter or reject messages, it just marks them (with an X-MailScanner-SpamScore:
line in the message header).
SpamAssassin checks for a long list of characteristics that are
associated with spam: for example, the word "viagra" is in the Subject:
line, the sender's IP address is on a blacklist of open relays, and so
forth. Each test that matches increases the overall "spam score" of the
message. A message with a score of 10 or more is very likely to be
spam, a score of 5 is more tentative, and anything lower than that
might be legitimate.
Check out http://www.spamassassin.org/tests.html for a
list of some of the checks SpamAssassin performs.
Filtering into a Spam Folder
The user's mail client or server can filter messages based on
the "spam score" generated by SpamAssassin. The easiest way to do this at
Dartmouth is to use the BlitzMail/IMAP server's built-in "Spam Folder"
filtering. BlitzMail users can set their Spam folder within the
BlitzMail program itself, in the "Folders" window
. IMAP users can go to https://dartmouth.org/spamfilt.html
In either case, the relevant settings are (a) the name of the folder
where Spam messages should go, and (b) the sensitivity of the filter
(how high the "spam score" must be for a message to be counted as spam).
Caution: No filter is perfect.
Always remember to check your Spam Folder periodically in case a
legitimate message has been inadvertently tagged as spam.
Filtering manually
If the built-in filtering offered by the IMAP server isn't sufficient,
some IMAP clients allow you to configure more elaborate filters. That will
allow you do things like rejecting messages from specific senders (regardless
of SpamAssassin's score). The mechanism for setting up your own filtering varies
depending on what mail client you use, but here's what you need to know about the
information SpamAssassin provides.
A typical header line generated by SpamAssassin looks like:
X-MailScanner-SpamScore: sssss
Note that there is one s in the header for each point of the Spam
Score; this enables you to filter for a Spam Score >= 5 by creating
a filter for "X-MailScanner-SpamScore" CONTAINS "sssss"
Filtering and Mail Forwarding
If you are forwarding, you may utilize Spam Assassin on a limited basis. The
added header "X-MailScanner-SpamScore: sssss" still appends to all incoming
messages. However, you must use the filters in the account you are forwarding
to. In Outlook Express it is called "creating a rule". This entails
"Filtering Manually" as described above.
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