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How do ISPs identify spam e-mails, to block their transmission to users?

How do Internet Service Providers (isps) identify spam e-mails, to block their transmission to users?
Rosemary Phillips from Surrey (Age 55+)


4 Responses

  1. Internet Service providers use special software to filter out spam emails. This software maintains a list of rules and tests that are applied to incoming messages to decide whether they are spam or not. Usually each message is checked against multiple rules and tests and the results are accumulated to come up with an overall score. Messages scoring higher than a certain threshold are deemed to be spam. The tests may look into various parts of the message. For example, the sender’s address may be compared against a list of known spam senders. The message subject or contents can be analysed for presence of known spam indicators, such as capital letters in the subject, images in the message body, certain words in the message text. Since spammers are constantly trying to come up with new ways to get around spam filters, the spam tests database has to be kept updated to detect new types of spam. This is quite similar to the way how antivirus software has to be regularly updated to detect new computer viruses. Maintaining the spam tests database manually is quite tedious and resource consuming. That is why modern spam filtering software often uses user feedback. Users can mark messages as spam and the software would keep this information in its database. All future messages can then be compared against past spam for similarity. This way the spam filter will gradually “learn” from the user to recognise new types of spam automatically.

  2. Thank you. I’d like a bit more detail about the “rules and tests” that are applied.

  3. These vary widely depending on the particular spam filtering software used. Simple examples could be:
    – check if the sender’s email in in the list of known spammers;
    – check if the message subject contains words in all capitals (spammers tend to use all-caps subjects often to attract reader’s attention to their emails);
    – check if the message text contains certain words (like “viagra”, “rolex”, “lottery” etc. associated with popular spam emails).

    Some filters allow users to mark messages as spam. These messages are then stored in a separate folder and new messages are checked for similarity to past spam emails. For example, if a new message contains largely the same words or phrases as a past spam message, then this new email is deemed similar to past spam and thus is likely to be spam too. This way a spam filter can detect new types of spam emails without the user having to formulate new spam checks manually.

  4. Hope this helps 🙂

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