A huge amount of marketeers do not fully understand how email spam filters work and that’s one of the main reasons why badly developed email campaigns fail. Understanding how spam filters work will eventually increase your deliverable percentage and hopefully keep your subscribers and clients happy. Let’s start of by listing the main spam filters used widely by large organisations and services:

  • SpamAssassin, a very popular open source spam filter.
  • MessageLabs, an expensive corporate server-side spam filter which is regularly updated.
  • Postini, server-side spam filter and owned by Google.
  • Barracuda, a hardware spam filter which is used by large organisations within their own datacentres.

Spam filters look at a list of criteria to work out if the email is spam or not, examples such as using caps in the subject line hugely increases your chances of being filtered out by spam filters. Each spam filter assigns points to every email received and if your campaign exceeds the threshold, your email will get filtered out. The threshold is different on each server depending on the user settings, the higher the settings the less spam the recipient will receive.

Common Mistakes

The following is a list of mistakes most marketeers make when developing their email campaigns. Avoiding these mistakes would greatly improve your email from being filtered by spam filters. Please try to avoid the following mistakes, please.

Using all CAPS

Using all caps in emails is not only rude but quite hard to read, especially if it’s a lengthy sentence. You don’t want to be this woman, fired for using caps in her emails.

Poorly coded HTML

Building email newsletters using poorly coded HTML is BAD,BAD,BAD!. Never convert a Photoshop or Microsoft Word email newsletters to HTML. Either learn how to code HTML the right way or hire a professional.

Few Images, No text

The amount of emails I see which is just one large image and with little or no text, makes me laugh. Spam Filters can not read images so automatically they filter this as spam. Use a combination of 50% images and 50% text and you should be good to go.

Using Test in the subject line

Happens to agencies all the time, sending out a newsletter to a client for approval only for the newsletter to be sent to the client’s spam folder which causes a lot of problems!.

Hopefully following my advice should help your clients/customers actually receive your newsletter and make us all very rich….only kidding.

about Mohammad Koubeissi

Freelance Web Developer - Tailor, Gym Rat and Mountaineer.

  1. 7 years ago

    ainslie johnson

    Thanks for a great post, i have definately fallen into one image all text category. Basically trying to hammer home my message to much in the email but its obvious really that I just need to focus on making an impact.

  2. 7 years ago


    great post, i needed this information a long time

  3. 7 years ago


    Thanks for that info . I’m guilty of all of the above !!

  4. 6 years ago


    Very useful tips, thank you. The point about CAPS is so logical, but I’ve been doing it all the time.

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