How do spam filters work?

Most anti-spam filters work by scoring email messages based on the email headers, subject and body text - some common triggers would include:

1. The subject line starts with “free” or contains FREE in caps.

2. The email contains the word “free” in certain phrases (free offer, free access etc.).

3. Certain words like “guaranteed” in capitals.

4. Words like “unsubscribe,” “leave,” and other list removal phrases.

5. Using font sizes that are +2 or larger - or equally very small fonts.

6. Background in an HTML email that isn’t white”.

7. Claims compliance with spam regulations or with US Senate Bill 1618 or House Bill 4176.

8. Urge you to call now or claims you can be removed from the list.

9. Phrases like 'what are you waiting for', 'while supplies last' etc.

10. Uses a Nigerian scam key phrase such as “million dollars”.

11. Money back guarantee.

It is not to say you cannot use these words - they are just likely to score worse than others and be 'more likely' that the email would be caught as spam.

You may ask - why tell people this - are you not just telling spammers how to avoid these. The answer is that they will already know - most spammers install anti-spam software and score their own messages to try and get them through spam filters!

On top of this many spam filters employ other techniques including blacklists and whitelists to help improve the accuracy of their filtering.