How to Prevent Your Newsletter From Being Unintentionally Blocked / Filtered

Don't let your legitimate ezine be the victim of spam mail filters

How to Prevent Your Newsletter From Being Unintentionally Blocked / Filtered

by Christopher Heng,
Written: 29 September 2000

Update (5 February 2012): please note that this article was written in 2000, back when I was still publishing newsletters. I have not tested Outlook's or Hotmail's current junk mail filters, since, as noted below, I no longer run such newsletters. As such, I have no idea what the various filters do these days.

Surprising as it may sound to some of you, it is possible for a newsletter to be unintentionally filtered by your subscriber's ISP, email service or even his/her own email software. Such blocking may be occur silently, in which case you would never know your newsletter was never delivered, or it may occur with the email service sending you a bounce message (rare).

I have experienced this first hand with another of my newsletters,'s newsletter. It came to my notice when I started receiving bounce messages from the mail systems of a few subscribers stating that my newsletter could not be delivered because it contained "offensive material" (or the like).

For those unfamiliar with's newsletter, it is an opt-in newsletter (like my other ezine, thesitewizard™ newsletter) that basically publishes links to free programming and webmaster resources that I discover on the web. By no stretch of imagination can it be said to contain "offensive material". What then could have triggered these mail systems' filters?

Update (5 February 2012): Before you email me to ask, please note that both these newsletters have been replaced by newsfeeds. You can subscribe to these newsfeeds free of charge directly from your browser without giving me any email address or anything like that. "Subscribing" in this context merely means getting your browser to automatically check my sites for updates. To do that, just click either's newsfeed or's newsfeed (or both). If you have a modern browser, it should prompt you for confirmation and do the rest by itself. See How to Subscribe to or Read Newsfeeds if you want the details of how it works.

With the help of one of the affected subscribers (the mail services themselves did not bother to reply to my queries), I managed to isolate the offending phrases in my newsletter that caused the hiccup. If you publish a newsletter yourself, or are planning to do so, you might want to note the following points to avoid your newsletter being unknowingly blocked by these mail services.

"Free" in Capital Letters

If you put the word "free" in capital letters in your newsletter, chances are high that it will trigger the filters of some email services and mail readers. This is a bit unfortunate for the newsletter since it only deals with free things and I had the tendency to put titles of sections in capital letters.

Lesson: don't use the word "FREE", where every letter in the word is in capital, not even in titles of sections.

"Free" Followed by an Exclamation Mark

The word "free" (whether in small or capital letters) followed by an exclamation mark (ie, "!") is also taboo. It doesn't look good for people who need to use that word, does it?

Lesson: "free!" and all its variations (capital and small letters) are taboo.

It's Not Just the Mail Services

Recently, when I complained of these email filters blocking my legitimately subscribed newsletters, some visitors informed me of other filters used by popular email software and email services:

  1. Apparently Microsoft Outlook also filters mail based on keywords like the above if the user enables a particular function.

  2. Hotmail has an option which purports to block spam by causing email that is not directly addressed to you to be filtered away. While it probably does fulfill its function most of the time, any legitimate newsletter that does not have the subscriber's email address in the "To:" field will also be filtered away. Since most mailing list services (especially the free ones) do not put the subscriber's email address in the "To:" field, it is possible that your subscriber will never see an issue of your newsletter if this feature is enabled.

Now I don't know how many people actually enable this facility in Outlook and Hotmail but you can be sure that somewhere around the world there is bound to be a few who do who also happen to subscribe to your newsletter.

Lesson: Alas, the only way that I can think of to work around this is to educate your subscribers in your ezine or newsletter sign-up page.

The Silent Killer

I only found out about such filters because some mail services bothered to bounce my newsletter with notes of explanation. But I wonder how many of my subscribers never received my ezines because it was silently filtered by either their mail service or their email reader. There's no way for me to tell, since if they don't bounce it, I won't know of it.

Your newsletter might be suffering the same fate if, like me, you used the same words that trip the filters. This might be a good time to fire up the search and replace facility in your editor and change those offending words before you forget.

It is possible that there are other words that trip the filters. I only know of those that frequently occur in my newsletters that are also blocked. If you know of any more, feel free to drop me a note.

This silent killer is designed to kill junk email (spam). Don't let your legitimate newsletter be its unintentional victim as well.

This article can be found at

Copyright 2000-2018 by Christopher Heng. All rights reserved.
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