Why does Thunderbird think my email is a scam?

By Ros Hodgekiss on 8th July 2010

We learnt a valuable lesson after sending our most recent monthly newsletter, which we’d like to share. It turns out that our friends using the Mozilla Thunderbird email client had their newsletters prefaced with the alert, “Thunderbird thinks this message might be an email scam.” It looked something like this:


Of course, we weren’t trying to scam anyone with our monthly dose of email tips and app updates… So what had we done wrong?

After a bit of tweaking, we discovered that Thunderbird systematically throws up this alert when it sees a URLs in your HTML email copy. This is also an issue that similarly affects Outlook and Entourage email clients, too. In this case, we had made the mistake of adding the following line:

If you would like to support the National Wildlife Federation in protecting wildlife and their habitats, kindly donate at http://killspill.org/.

Big boo. Although adding a text link doesn’t explicitly set off any spam filters (ie. your email will still get delivered), the alert could potentially be enough to put off some readers. When we changed this line to…

If you would like to support the National Wildlife Federation in protecting wildlife and their habitats, kindly donate today.

…Thunderbird calmed down. Well, that’s something we won’t repeat.

Another approach is to append your links with cm_dontconvertlink, as phishing alerts are often thrown when the URL in the email copy doesn’t match the URL (<a href="...">) it’s linking to. Adding cm_dontconvertlink tells our app to not make your link trackable (ie. leave it alone), so this discrepancy doesn’t arise. Here’s an example:

<a href="http://www.myserver.com" cm_dontconvertlink>this is a link</a>

The obvious downside is that clicks on links like this won’t be recorded in your campaign reports.

The moral of the story is, don’t type out URLs in your email copy, or be prepared to turn off tracking if you wish to do so. Otherwise, the bird might just drop some of its thunder on you.


  • evermeg
    8th July

    Similar types of “fraud alerts” also occur when you place a url in the text in some versions of outlook and entourage.

  • Justin
    8th July

    Your post didn’t make this clear, but I assume it’s happening because of link tracking. The link’s text is a URL that doesn’t match where the href is pointing, and that’s what sets off the phishing filters.

    So go ahead and use links in your email copy — if those links have href attributes that match the link text. And if you’re using any form of link tracking, they probably don’t.

  • Paul Flynn
    8th July

    “When you bring the thunder sometimes you get caught in the storm” Kenny Powers.

    Good tip and useful input from Justin.


  • Ros Hodgekiss
    8th July

    Justin - Excellent point, you can turn off link tracking by adding cm_dontconvertlink to your links like so:

    <a href=“http://www.myserver.com” cm_dontconvertlink>this is a link</a>

    I’ll update the post to mention this. Cheers!

  • Nickolas Simard
    9th July

    You could always include a visual representation of the URL (ie. an image or logo) you want to link to in your design.

    In the latest newsletter design for the company that employs me (lucky them ^_^ ) we added our website address into the header of the email, and set the link (a trackable one) to our website…

    No phishing alert, since the URL text is an image.

    Should you want to see it, you can always check the HTML version there: http://bit.ly/clMbWB (Geez, that sounds like self-promothing… sorry about that)

    In any cases, thanks for the tips on placing URL in text into the content of a newsletter. I shall not do so (except with cm_dontconvertlink)!

  • Rob
    17th July

    We have always sent our mailer out with the URL in the text, we first discovered this two years ago when we started checking our mails with Spam Assassin .

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