Spam Filters

If you send email campaigns for long enough, you’ll eventually run into spam-filter issues. Innocent email marketers who send permission-based emails to people who requested them still get spam filtered all the time. Unfortunately, there’s not a quick fix. However, understanding how spam filters work sure helps. Here are some examples and tips.

Bayesian Filtering

This is one of the most important spam filters to learn about, since it’s installed in so many email applications (Outlook, Mozilla Thunderbird, and Apple Mail, for example). Bayesian filters work by watching users classify email as junk (such as when they click a “this is spam” button). It reads the junk mail, compares it to other emails the readers called junk, and looks for common traits in the subject line, the content, the hyperlinks, the sender, and so on. Over time, Bayesian filters learn to scan for those traits in every email message. And every time they find something that looks spammy, they assign it a score. For instance, using “Click here!” might get you 0.7 points. Using bright red fonts might get you 2 points. Including the word “mortgage” might get you 1 point, while using the word “Viagra” might get you 5 points. Once an email exceeds some threshold (set by the user), the email is classified as spam and thrown into the junk folder. It’s amazing how many different things Bayesian filters look for.

Email Firewalls

Managing email servers—and incoming spam, viruses, and phishing attacks—can be a lot of work. So large corporations usually install email firewalls to handle their incoming email. (Google the terms “Barracuda Firewall” or “Postini” for some examples.) Think of them as spam filters on top of spam filters. They’re big, heavy-duty gatekeepers, and they’re not friendly at all. They often use a combination of Bayesian-style/adaptive filters, community reporting, blackhole lists, and a little bit of proprietary magic to keep spam out of the company. Most of the time, when your email’s not getting through to a larger company, it’s their firewall. You can think of these firewalls as kind of Xenophobic and paranoid. They’re all twitchy, and tend to ask questions like, “Okay, is this sender new to me? Why is he sending copies of the same, exact email, to a bunch of people in our company? Spammers do that kind of stuff. How long has their server been around? Can I really trust this sender?”

Spam firewalls are usually only a problem when you first start sending campaigns to a big client. You’ll experience some deliverability issues in the beginning, because you’re new. They’ll eventually learn to let you through, though. To expedite things, you may have to ask the IT people in charge of the firewall to whitelist your IP Addresses (or the IP address of your email service provider).

Challenge/Response Filters

These are more common among at-home recipients. When you send email to someone with a challenge/response filter, here’s what happens: If you’re not already in that person’s buddy list or address book, then you’re considered a stranger to him. And if you’re a stranger, you could be a spammer. So their challenge/response filter sends you an automatic reply with a question that you have to answer, or some link you have to click (this proves you’re a human, not a spambot).

Remember that you have to be whitelisted if you want your emails to get through. So when people fill out your opt-in forms on your website, ask them to “please add our email address to your address book.” Use your opt-in process as a way to set expectations and get whitelisted up front. Whenever you send a newsletter, make sure the reply-to address is valid, and that a human checks it after each campaign.

Tactics for Avoiding Spam Filters

Now you know how a lot of anti-spam systems work. You really have to think like a spam filter when you design your emails. Just like you think like a search engine when you design web pages. Spam filters read your emails and look for similarities with known spam. You don’t want to do things that’ll get you accidentally thrown into the junk folder.

Here, then, are some tips for avoiding spam filters:

  • Campaign metadata: Spam filters want to know that you’re acquainted with the person receiving the email. We recommend using merge tags to personalize the To: field of your campaign, sending through verified domains, and asking recipients to add you to their address book.
  • Your IP address: Some spam filters will flag a campaign if anyone with the same IP has sent spam in the past. When you send through MailChimp, your email is delivered through our servers, so if one person sends spam, it could affect deliverability for our other users. That’s why we work vigilantly to keep our sending reputation intact, and it’s important that all users abide by our Terms of Use.
  • Coding in your campaign: Spam filters can be triggered by sloppy code, extra tags, or code pulled in from Microsoft Word. We recommend using one of our templates or working with a designer.
  • Content and formatting: Some spam filters will flag emails based on specific content or images they contain, but there’s not an all-encompassing set of best practices to follow or things you absolutely need to avoid. But, we do have a few recommendations.
    • Design your campaign to be clear, balanced, and to promote engagement from your subscribers.
    • Make sure your subscribers have opted-in to receiving your emails.
    • Be consistent. Try not to stray too far from the content and design that your audience already associates with your brand, website, or social media channels.
    • Test, test, test! Use A/B or Multivariate Testing to learn how changes to your content affects delivery and engagement.