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Context filtering
Context filtering is an
anti-spam /
mail policy method that does not deal with the contents of the mail
but rather uses the context of the
SMTP
connection to decide whether a mail will be accepted or not.
This method usually prevents reception of an e-mail in the first place; thus,
common anti-spam features like quarantine, redirect or delete can not be
applied. This method also distorts statistics of anti-spam programs because it
is usually unknown how many mails (Spam or otherwise) would have been received
through a certain connection.
Filtering by context includes methods like
DNSBL lookups,
domain or IP blacklists, etc. This method has an extremely low rate of false
positives and false negatives.
On the plus side, this method is usually saving
bandwidth
and server utilization.
The most common context filter is DNSBL filtering. The efficiency of this
method depends on the blacklist used.
For high volume mail sites it is recommended to setup local copies of the
DNSBLs of your choice and fill them through DNS zone transfers. This reduces the
permanent traffic through external DNS lookups. Some DNSBLs also allow only a
certain amount of queries by day for every IP address
to keep their expenses on bandwidth low.
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This guide is licensed under the GNU
Free Documentation License. It uses material from the Wikipedia.
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