Spam Filter FAQ
Oct 02, 2003
Q: What is this spam filter FAQ about?
A: This page explains why I might not have replied to your email and, instead, puts the blame on my spam filter. ;-) It also tells what you can do to make sure I get your email, and what makes my spam filter think that your message was an unsolicited commercial email. It offers tips on what common message characteristics you should avoid, so that other people's spam filters will not block your legitimate messages.
Q: Why am I seeing this page?
A: Probably because you clicked on a link to this page from an email I sent you. I might have included a link to this page in my email signature.
Q: So, why do you use a spam filter in the first place?
A: Due to the huge amounts of spam I receive every day (> 500 messages a day) it has been impossible to manually read every email and decide if it is spam. Therefore, I switched to using a spam filter (SpamAssassin, a free product) sometime ago. For a long time after that, I had configured it so it would only mark each suspicious message with "[Spam]", but did not delete such emails automatically. I also use the junk mail controls built into the Mozilla Thunderbird email client program.
I used to filter these emails to another folder, and whenever I had some free time, I checked that folder on a regular basis.
Q: What do you do now, that made you write this FAQ?
Since around May 2003, it has been entirely impossible to inspect my Spam folder to weed out the good from the bad. So I began to delete all such emails without looking at every single one. There are only 24 hours in a day, and it has become humanly impossible to do this process once every day.
Also, the spam filter I use has been quite accurate in identifying spam, hence the number of legitimate messages that get flagged as spam ("false positives") has reduced drastically to around 1 in every 200, thus making the whole process not worth the time.
Q: How will I know if my email was deleted by your spam filter?
A: The short answer is, if I do not reply to you within a reasonable amount of time, you should assume that your message was inadvertently deleted. I apologize for having had to do this, but as you've seen from the questions above, there is no other alternative.
Q: What should I do so that my emails are not flagged as spam?
A: I can suggest a few rules that SpamAssassin uses to identify spam, so that you may make sure your messages do not satisfy them. That should help get your email to pass through most other spam filters too, so if you have been having trouble sending email to people in certain organizations where spam filtering is enforced, you have come to the right page.
Q: Just tell me what I need to do:
A: OK, let's start. I will list here the rules, beginning with those that are most easy to implement and progressively to those that are more difficult to. Before I begin, I need to tell you about how a message is flagged. SpamAssassin performs each of these tests on each message received, and assigns a score (positive or negative) for each test. Then, it sums up the score for each message, and if it is beyond a particular threshold, it flags it as spam. So, if your message satisfies two or more rules from the list below, there is a higher likelihood of it being classified as spam.
| Rule | Why it is treated as spam | What you should do |
|---|---|---|
| HTML Email | When you send email formatted as HTML instead of plain-text, SpamAssassin thinks it is spam. The more colors, images and fancy text you have, the more it thinks it is spam. | Try to configure your email client to send all messages in plain-text. It also requires less space in the inbox if you use only plain-text for email. You will not be able to have nicely formatted text such as bold or italic, but if your message is not at all received, who cares about the formatting? |
| Yelling (writing text in ALL CAPITALS) | When SpamAssassin detects a full line in capital letters (considered 'yelling') it assigns it a positive score (more likely to be spam.) | Do not use ALL CAPS in your email. If you really need to yell at me, please take a number and wait in line for your turn. ;-) |
| More lines of yelling | The more lines you have in all-capitals, the higher score your message gets. | Same as above, write everything in usual sentence case. It is also good netiquette to not write everything in all-capital letters. |
| To: is empty | If you do not address the message specifically to me, or anyone in particular, there is reason enough to believe it is spam. | Always put the intended primary recipients' names in the To: field, and the secondary recipients' names in the Cc: or Bcc: fields as appropriate. |
| "Undisclosed-Recipients" | If you (or your mail client) enters "Undisclosed-Recipients" in the To: field, it is 99% likely to be treated as spam. | Make sure your mail client does not do things like this! |
| Subject has a unique ID | If you include a unique ID (e.g. GD93jij83) in the subject line, this tactic is commonly used by spammers to check which email accounts are active and which are not. | Have a simple, though meaningful, subject line. Do not include weird-looking numbers. |
| Subject is in ALL CAPS | Same as for the lines in all-capitals in the body of the message. | Refrain from all-capitals in the subject line. |
| you@you.com or similar | If you send an email with a subject like this, it is obvious that it is fake! | Use real names or saved aliases while sending email. |
| Sent to too many people at once | If you send one of those ubiquitous "forwards" to everyone on your address list, it is probably not an important email. | Do not send forwards. They only waste valuable time, money and Internet bandwidth. If you do need to send important announcements to all your friends, put their names in the Bcc: field instead of in the To: or Cc: fields. |
| Recipient List is sorted by address | Spammers usually sort their lists alphabetically. | Do not include too many recipients in the same message. |
| Typical spam features | Things like the Nigerian spam, messages offering free stuff, guaranteed results, investment suggestions, full refund, free trial, 100%, call now, direct email marketing, university diplomas, penis/breast enlargement, mortgages, money-back guarantee, porn site advertisements, remove now, unsubscribe, complies with Senate bill so-and-so, claims that you asked for this email, or you willingly subscribed to this list, opted-in, or provided permission to be spammed | Do nothing. If your email contains any of these terms, I am not gonna look at it anyway. |
A novel way of almost guaranteeing that your email is never flagged as spam, is to incorporate Habeas headers into your message. For more information, please check out their FAQ page. It is free for personal use, hence a good choice for the cash-strapped individual email user like me.
Disclaimer: This is not an official or exhaustive document. The SpamAssassin tests page was used as a reference, and that page should be consulted for any clarification of these rules. This FAQ is intended for the common man concerned about his/her email being flagged as spam.
