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Spam Filter Basics: How to Keep Your Emails from Getting Blocked

Article Outline

e-mail spam closeupIt’s the holiday season, which means marketers are really busy. B2C marketers often see a majority of the year’s sales now, and B2B marketers are concerned with making the year’s target goals. There’s a lot of email flying around, which means a lot of competition for getting your messages read. The last thing you need is to discover that your emails are getting caught in a spam filter – never getting the chance to be read by that hot prospect.

The fact is, without proper adherence to best practices, the email you and your team worked so hard to craft could go unread in the spam folder. The best way to avoid this is to understand what spam filters are and why they’re necessary. Your best strategy to improve inbox placement is not to trick the spam filter – it’s there to protect the potential recipient. Rather, your strategy should be to create content that’s valuable to your customers and prospects and tailored to their interests, so they welcome your email.

How do Spam Filters Work?

You may have already encountered webmail clients with rigorous spam filters like Gmail and Outlook, or some of the many other powerful software and firewalls such as Cloudmark, Proofpoint, MessageLabs, SpamAssassin, and Barracuda. These filters all basically follow the same concept: they evaluate each email they receive against certain criteria. These criteria are unique to each program and also customizable on a local level by the recipient. The filter will assign points to factors they consider “spammy” and weigh these toward your email’s spam score. If your email exceeds their threshold, it doesn’t make the cut and gets sent to the junk folder – or worse it gets blocked, which means it never reaches the intended recipient at all.

The list of criteria filters check for grows and changes all the time as they learn from user activity and share information with each other. You can find a detailed list of the tests SpamAssassin currently performs here, but the tests fall into a few general categories:

  • Engagement
  • Reputation
  • Authentication
  • Formatting
  • Content and word triggers

Let’s take a look at how these categories impact deliverability in the average inbox.


Internet service providers track how engaged subscribers are with an email and its sender, as well as the nature of that engagement. Positive engagement indicates a low likelihood of spam, and negative engagement can mean trouble for email senders. Positive actions may include things like opening a message, adding an address to their contact list, clicking through links, enabling images to display, and scrolling through the message.

Negative actions generally include reporting the email as spam, deleting it, moving it to the junk folder, or ignoring it. Engagement ratings are a good reason to use only opt-in email marketing lists. Opt-in maximizes the likelihood of engagement, because you’ve already got an established relationship with the receiver, one that they’ve actively agreed to.


A sender’s reputation is arguably one of the most important factors for passing a filter’s tests. In fact, according to Return Path, over 70% of a sender’s score is based on their reputation. Spam filters are constantly collecting data on the emails they receive and the sender they receive from, and sharing their aggregate data within the community of spam filters. They want to know whether senders are on global white lists or whether their domain or IP is blacklisted, whether a sender has been sending to honeypot emails (email addresses intended to catch spammers), or whether they have a large number of spam complaints from the users they are emailing.

Spam filters also weigh how long ago the domain was created, as well as when the domain is set to expire. That means building up your domain’s reputation can take time, patience, and perseverance. However, if you keep these factors in mind, you’ll help keep your reputation clear – and your emails in the inbox.

Remember that when you insert third party URLs into a message, those links carry their own reputations as well. They can be flagged by spam filters, which may cause delivery and performance issues for you. For example, if you’re sending a travel newsletter, you might include special offers from an airline, a hotel, and a rental car company. If one of them gets flagged, the content and links from that business could affect the filtering of the others.


STOP EMAILThe first step to developing a good reputation is proving who you are. Your customers want to know that the email they are reading is really from who they think it is. They also want to feel secure that their information and identities are not under attack. There are many methods for email authentication that a spam filter looks for in order to protect users from phishing scams – the kind of fake emails that could cost email recipients a lot of money and grief. The standard is the Domain Keys Identified Mail (DKIM), which verifies that the third parties sending an email are authorized to send on behalf of that domain through their system. (This includes email service providers such as Act-On.) A message that does not pass could be considered phished or spoofed, which means it gets sent to the spam folder or blocked.

DKIM is relatively easy to implement, so most senders comply with it, but it’s not the most secure option. Increasingly, senders are moving toward other authentication solutions such as Sender Policy Framework (SPF) and Domain-based Message Authentication, Reporting and Conformance (DMARC). These email authentication methods make it harder to forge an identity and therefore they’re more secure for your marketing initiatives and your brand.


You’ve probably spent a great deal of time considering how your email’s design will improve clickthrough rates. But have you considered the impact design and formatting might have on your spam score? There are certain triggers that could weigh against you. Filters like to see that there is a text and HTML version of your email available and that they match each other. They hate sloppy code – legitimate senders should be testing their email formatting, so sloppy code is a dead giveaway that the email was sent by a spammer. Code pulled from Microsoft Word (which happens when you copy and paste content from a Word document into the email template) should also be avoided, since it’s incompatible with most mail clients and filters will weigh it against you.

As far as design goes, there are several things filters like for. Elements like overly large font sizes, or text in obnoxious colors like bright red, lime green, or hot pink might raise a flag. An unbalanced ratio of text to images may also trigger a spam filter and count against you. Using one large image with no text is generally unwise since it’s a spam indicator. Plus, if the user (or the email client, such as Outlook) has images blocked as a default, they won’t see your message.

Content and Word Triggers

You can find many lists of words that supposedly trigger spam filters. All of them warn against using any variation of the word “free,” words in all caps, too much punctuation, or common spam phrases such as “low-interest mortgage,” “casino deals,” “cheap prescriptions,” and so on. These words and phrases change often, and while they should be considered when you’re creating a campaign, they won’t necessarily send your email to the spam folder.  What is most important is relevance and recognition. Remember that the criteria spam filters use to assign your email a spam score is sophisticated and well-rounded – using the word “free” in a context that is appropriate will not send your message directly to the junk folder. If your campaign or product calls for these trigger words but your email is otherwise clean, you don’t have to be afraid to break some of the rules.

Are You Getting Filtered?

It’s not always immediately evident that your emails are being filtered and not reaching the inbox so it’s important to monitor campaign metrics. An increase in spam complaints is not only a sign that your campaign was not very well-received and either needs some tweaking or needs to be more targeted, but will affect your reputation and could lead to lower deliverability in the future. A decrease in open and click rates is a good indicator that emails aren’t making it to the inbox and are most likely being filtered/blocked.   As a marketer, regardless of the time of year or campaign being run, all of the above hold true.  Filters are constantly evolving and therefore it is imperative that marketers do as well.

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