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In this informative guide, we’ll give you a working knowledge of SPAM filters, how SPAM filters work, and how to maximize your chances of avoiding them.

What is SPAM?

As corporate email marketers, it is vital that we have a solid understanding of what SPAM is how it affects our businesses, and the implications of SPAM prevention on any email campaigns we send. 

Simply put, SPAM is unsolicited (unrequested) commercial email.  This means sending someone (who didn’t request it) an email that promotes your product or service.

Why Do SPAM filters exist?

SPAM is extremely costly to the networks that carry it, and to the intended receivers.  It is estimated that unsolicited emails cost ISPs around $20 billion per year*.  It is also estimated that about 99.99% of the email accounts that exist are using some type of spam filtering technology.  SPAM is costly because it takes time and resources to deliver, process, and store the email, just like any valid email message– it requires resources to move it from one location to the next.  Multiply that process by 10 billion times and it’s a lot of time and money. In the case of malicious SPAM (which carries harmful viruses, or seeks to defraud recipients of confidential information), those affects can be far more costly. So in a nutshell, SPAM filters were created to divert as much of these unsolicited, potentially harmful messages as possible.

* "The Economics of Spam," by Justin M. Rao and David H. Reiley

Critical Impact’s Position on SPAM

We take SPAM and deliverability very seriously at Critical Impact.  Helping our customers deliver email messages to the inbox is our livelihood. Our clients come to us to get the very best guidance and partnership available for optimizing this process.  That’s what we do.  So, we’ve got plenty to share with you when it comes to avoiding Junk Folders, and all the reasons why, if you aren’t careful, you  you might end up there.

Know the CAN-SPAM Act and Adhere to It

In this article, we’ll tell you exactly how to manage this entire process on your own.  Although you don’t have to take our advice.  If it all seems like too much to handle or too time consuming, get in touch with us and we’ll do it all for you.

The CAN-SPAM act applies to sending mass emails of a commercial nature, where those emails are intended to advertise or promote a product or service. Although the CAN-SPAM Act includes a rather extensive set of guidelines, the most important elements of it are summarized below. 

To become CAN-SPAM compliant, you must adhere to the following guidelines* :

1.Identify your email message as an advertisement or promotional mailing.
2.Do NOT use false or misleading header information.
3.Include your mailing address.
4.Include instructions on how recipients can opt-out of future mailings from you.
5.Within 10 business days of receipt, honor all opt-out requests.
6.Do NOT use deceptive subject lines.
7.If you are using a third party to send your emails, carefully monitor their activity because ultimately, you are responsible for their actions.

* Disclaimer, the guidelines above are not legal advice.  Please seek professional counsel from an attorney to if you have any specific questions or concerns concerning your legal compliance.

Adopt Permission-Based Email Marketing

One of the most critical factors in determining your ability to avoid the Junk folder, or any  SPAM filter for that matter, is your adoption of a “permission-based email marketing” mentality. 

What is permission-based email marketing?

In essence, this means only sending emails to those individuals who have specifically requested email communications from your company.  To be clear, permission-based email marketing does not include sending emails to individuals who have requested information on a similar subject matter from a third party – this means expressly from your company. 

Permission-based, or opt-in email marketing, means potential subscribers find you, and provide their email address in return for your email messages.

For clarification, there is a second category of email messaging known as “transactional email”, which is essentially defined as emails that are sent to a customer with important information about their account or service, e.g. relationship emails. CAN-SPAM laws, in general, do not apply to transactional emails. It is generally advised to apply the same guidelines mentioned in Section 2 for all transactional emails as well, but the CAN-SPAM act does allow for some flexibility with respect to transactional emails.

If you are a business that sends email to foreign countries, you have the added burden of needing to be familiar with the laws that protect would-be email recipients in those countries as well.  Canada, for example, has a very different set of SPAM regulations vs. those in the United States.  The SPAM laws in Europe also vary from country to country. Before you begin sending emails to other countries, take the time to familiarize yourself with those laws as mistakes can be costly.

* Disclaimer, the comments above are not legal advice.  Please seek professional counsel from an attorney to if you have any specific questions or concerns concerning your legal compliance.

Spam Triggers

Broadly speaking, spam filtering happens for two reasons – either the content of your message has been flagged by the recipient's email system as SPAM, or, the network characteristics of your email message ( the IP address it originated from, the domain it was sent from, etc.) has been flagged as a sender of SPAM, thus causing your message to be assumed as SPAM.  We will take a closer look at both of these scenarios and provide suggestions for both.

SPAM Filter Keywords and Phishing Phrases

Spam filters are highly protective of their algorithms and techniques used to either pass or “fail” (filter to junk) an email message.  They do this to ensure that spammers don't figure out their techniques.

There is typically not one simple change that you can make to cause your email to pass through a spam content filter.  Instead, most filters use a complex scoring system, and if your email  scores high enough, they treat it as SPAM.  An "all caps" word may be worth one point, repeated words may be another 1 point, including ‘$$’ in the subject line may be 5 points.   Once your message reaches a certain score, it will be treated as SPAM.  There are 100's of spam filters, each with their own unique settings and techniques.

We’ve compiled an extensive list of possible spam content and formatting pitfalls to avoid when creating your email messages. These are just guidelines and do not guarantee success.

Potential IssueDescription

Spam-like words

Free, guarantee, credit card, cash, credit, sex, enhance, etc.

Red text

Red is a loud color and can be hard to read. It is also a spam tactic that may trip an email filter.

All capital letters

Resist the temptation to use capital letters and over-punctuate. When you use all capital letters, there is no differentiation in your words. This actually makes them harder to read. It also comes across to the reader LIKE YOU ARE SHOUTING, makes your email look like spam and will greatly increase the likelihood of your email being filtered.

Excessive punctuation

!!! or ???  This is likely to trip email filters especially when used in conjunction with spam-like words and capital letters.

Incomplete information

Your physical address is required by law. Always include your reply email address and your Web site URL, if you have one. Depending on your business, you may decide to include your phone number as well.

Excessive use of "click here“, especially in all capital letters

Make your call-to-action links more specific to avoid filters.

Excessive use of $$, and other symbols

Again, this tactic is likely to trip email filters. Use just one dollar sign for currency and use descriptive words instead of symbols to get your message across.

No "From:" address or noreply@

It looks like spam and will increase the likelihood of your email being filtered.

Misleading (or missing) subject line


Words and phrases to avoid in  subject lines (This does not include any inappropriate subject lines):

All caps
dollar amounts
Monetary reference
Wipe out/ Remove
get (?:rid| out) of| eradicate)
(?:owe| debt| obligation)
(?:alert| response| assistance| proposal| reply| warning| noti(?:ce| fication)| greeting| matter))
\bsecurity (?:[a-z_,-]+ )*?measures?\b/i
Dec)\S* \d+% OFF/
"At No Cost"
Cheap in Caps,  Money bonus!, low rates, will help,
starts with Do you dream,have,want,love, etc.
starts with Lose
line starts with Buy or Buying or dollar amount

Other Important Email Formatting Considerations

Plain-Text Version

Most email marketers are focused on sending the most professional, image-laden email they can dream up – but what about the plain text version?  Avoid the mistake of assuming that the plain text version of your emails is not important.  While most email platforms support HTML messages, they also support plain text.  Ignoring the plain text version is a mistake, and often a leading cause of getting filtered as SPAM.  Always include both versions within your email message.

Image to Text Ratio

The image to text ratio within any given email message is reviewed by spam filters and can add to the likelihood that your message may be filtered.  We advise our clients to avoid sending any image-only emails.  If you must include images in your email, plan carefully to avoid using more than 1 image per two lines of text.


In any sort of large scale mailing, it is considered a best practice to link to attachments (for download) vs. attempting to email the attachments directly to the recipients.  Your message has a much higher probability of getting flagged as SPAM, or worse, triggering a virus scanner.  So, avoid the hassle and just upload the file to your email server (or ours), and then provide a simple link to view or download the file.

Optimizing the Network Characteristics of Your Messages

Avoiding Blacklists

Blacklists can mean huge problems for businesses.  You’ll want to avoid these at all costs.  If your domain name or IP address are “black-listed”, any potential recipient’s mail server will see this and most likely, completely block your mailings.  Consider using a free service like to monitor your IP and/or domain name for blacklisting.

IP Reputation

The trustworthiness, or “reputation” of the IP address that you send emails from will directly determine how your emails are treated by recipient’s SPAM filters , e.g flagged as potential SPAM or otherwise. We strongly recommend using a free service such as to regularly check the “reputation” of your sending IP address.

5 Actionable Ideas

  1. Comply with CAN-SPAM!

    • If you play by the rules, your email campaigns will enjoy noticeably better results. Here’s a brief recap:
    • Identify your email message as an advertisement or promotional mailing.
    • Do NOT use false or misleading header information.
    • Include your mailing address.
    • Include instructions on how recipients can opt-out of future mailings form you.
    • Honor all opt-out requests.
    • Do NOT use deceptive subject lines.
    • Monitor the sending activity of those who email on your behalf.

2. Use a Permission-Based Approach

    • By requiring permission from your subscribers (before sending them an email), you are avoiding a whole host of potential SPAM filtering issues down the road.  Recipients who actually want your emails will be far less likely to report them as spam.  This also goes hand in hand with sending valuable, relevant emails – the better your content, the more subscribers you'll attract and the more engaged they will be over time.  Engagement rates are reflected in the total number of unsubscribes and bounces of time.  Both of which impact deliverability.

3. Set up Authentication

It’s easy to set up email authentication protocols (DKIM, SPF, Sender ID and Domain Keys)  - you just need to make sure your email server supports this technology.  It can make the difference between an email reaching the inbox, or getting treated as SPAM.  If you are uncomfortable in setting up email authentication on your own, contact your local IT support rep, or your Critical Impact rep and they can help walk you through the process. Even if you can’t use all 3 methods, at the very least get your SPF record setup.

4. Review your Content and Network

Do a complete review of your email messages to insure:

    • No known SPAM keyword-triggers present
    • You are using  HTML and plain-text  versions
    • You are not using attachments
    • You are not using emails that are image-only
    • Emails that do contain images, also contain an appropriate amount of text
    • Test your emails with a SPAM check service  (Critical Impact includes this)
    • Check the sending IP of your email server using any number of free services, e.g.

5. Start Working on your Reputation

As we said, building an positive email reputation takes time, but one bad mailing can ruin it.  So, get started now and make some decisions that will positively impact your deliverability in 1 months, 6 months, 12 months.    Review your policies on subscriber collection, list management, treatment of unsubscribes – check out your own SenderScore®, and take inventory of what emails you’ve sent in the last 3-6 months.  Are these mailings valuable, and relevant?  Are your open and unsubscribe rates trending upward or downward?  Answering these important questions will help you get started on improving your overall deliverability score and getting to the inbox


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