Spam filters
According to a recent survey, done by an email marketing
service provider, seventeen per cent of permission-based
email messages get incorrectly blocked or filtered
by the top twelve Internet service providers (ISP).
Even though you never had spammed or have no intention
of doing so in the future, your legitimate emails
might be filtered along with spammers sent emails.
It has become
important that you have a general understanding
on some filters currently in use to block the
passage of spam emails. You do not need the knowledge
to crack the filter cordon but to reach your opted-in
recipients who are interested in your product
or service information.
Filters usually
are located at mail server or ISP and on the intended
recipient's computer.
Content filters
If you are using words like free, win, you might
end up being blocked. Depending upon the content
of email, the content filters apply their standards
to determine whether the email should be part
of a recipient's inbox or it is trash. ISP's often
employ content filters created internally or adapted
from others. Content filters scan for a variety
of red flags.
The standard varies from one content filter service
to another. Microsoft Outlook's filter simply
searches for offensive keywords and key phrases,
or sometimes the content filter may be formatted
to scan all capital letters or non-standard colors.
Some content filters also check that the content
and tone of the subject line matches that of the
body or some filters will look at header information
to ensure that the email headers contain legitimate
and accurate information, rather than forged message
code.
Distributed
content filters.
There are many anti-spam companies that help Internet
Service Providers (ISPs) and corporate houses
handle deluge of spam e-mails. These filtering
systems employ intricate content analysis heuristics
that scan message content and create message signatures
that are distributed among the filtering company's
customers.
Blacklist
filters
There are many organizations who maintain list
of known spammers and these are address lists
that contain lists of IP addresses or domain names
of known spam sending systems. If the email coming
in matches a blacklisted provider, then the email
is blocked
Whitelist
filters
Whitelist filters check each message coming in
against a list of senders that are in the allowable
list. They allow those emails that are part of
the recipient's white list (allowable list) and
reject all emails that are not on the acceptable
email list.
Challenge/Response
filter
This is said to be an extension of the whitelist
filter. The recipient sets up a whitelis (allowable
list) and then whenever an email is received which
is not present in the whitelist data, a challenge
is automatically returned to the sender. The sender
of the email must reply to the challenge in order
to be accepted and allowed in the recipient's
inbox. The logic behind this is that spammers
use automated tools to send their spam which are
unable to reply to the challenge.
Pattern-matching
Filters
This filtering method consists of pre-defined
spam filtering. They filter out spam by searching
for banned words, text strings and character sets
in the email content, sender and subject line.
This method may result in large number of false
positives because genuine emails may unintentionally
contain banned words or text strings.
Bayesian
filtering It is an effective spam filtering technique
that uses statistics to analyze whether emails
are likely to be spam. The filters analyze both
the legitimate mail as well as the spam to calculate
the probability of various characteristics appearing
in spam, and in good mail. When a new email arrives,
it is analyzed by the Bayesian spam filter, and
the probability of the complete message being
spam is calculated using the individual characteristics.
| 
