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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.

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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.

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What is Blacklist?  
Types of Blacklist?
How did you land in a Blacklist?
How do Blacklists work?
Delisting from Blacklist? 
How to avoid Blacklist?
(Good email marketing strategy?)
Email Marketing Do's and Don'ts?
Open Relay
Proposals offered to nip the spam bud?
Spam filters?
Spammer's notebook?
Synopsis of Can-Spam Act
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