SPAM Filters

How SPAM Filters Affect Email Deliverability

The commonly accepted definition of SPAM is Unsolicited Bulk Email (UBE), or messages sent to a large number of recipients without permission. But recipients have evolved that definition through the use of their SPAM button. If they believe an email is unwanted and sent without permission, pressing the SPAM button instantly informs their mailbox provider. Clearly, the definition of SPAM is in the eye of the beholder. And if enough beholders view your email as SPAM, it will negatively affect the deliverability of future emails.


Gary Thuerk The History of SPAM – On May 3, 1978, Gary Thuerk, a marketer for the Digital Equipment Corporation, blasted out the first SPAM message.  Sent to 400 of the 2,600 people on ARPAnet, the so-called “first Internet,” the message invited readers to the introduction of DEC’s new DecSystem computer family.  Although considered a “flagrant violation” of the use of ARPAnet, DEC supposedly sold $13 million in new computers from the message.



To protect their subscribers, mailbox providers analyze both the sender and the content of incoming messages to determine if the email merits inbox delivery. Emails that don’t pass this scrutiny don’t get delivered. It’s that simple.

Unfortunately, inbox delivery statistics from SPAM filters don’t get passed back to email marketing platforms. Marketers never see them unless they subscribe to inbox placement data from an email consumer network or SPAM feedback loops. But the effects of filtering become evident when open and click-through rates start to decline precipitously.

While there are countless articles on how to avoid SPAM filters, their advice begins with a false premise. You can’t avoid SPAM filters. They are an integral part of the delivery process. If you send an email, it will get filtered. But if you learn how filters work, you can create emails that can pass through them unscathed, thereby increasing your email deliverability rate.

How Email SPAM Filters Work

Generally speaking, mailbox providers look at three principal aspects of mail when making filtering decisions – the source, the sender’s reputation and the content of the email.

Email Deliverability Filters

Source filters look at factors such as sender authentication, sender permanence and the age of the sender’s IP addresses and domains to identify potential spammers. Mailbox providers treat emails sent from new IP addresses and domains with caution. Longevity is key. Changing IP addresses and domains less frequently establish sending permanence, thereby improving delivery.

Reputation filters use algorithms to look at parameters like complaints, volume, spam traps, blacklists and message composition to generate a numerical reputation score. Mailbox providers then make filtering decisions about email coming from a sender based on the strength of their reputation score. Higher scores mean better inbox deliverability. Lower scores block or send your email to the recipient’s SPAM folder.

Content filters analyze every part of an email, including the header, footer, code, HTML markup, images, text color, timestamp, URLs, subject line, text-to-image ratio, language, attachments and more.  Their objective is to improve the user experience.  While individual content filters vary, you can take steps to improve the chances your messages will pass, including:

can you buy Lyrica in canada Balance Text and Imagery. Don’t create messages as a single large image. Doing so is a common spammer technique and also slows an emails server’s ability to process mail. SPAM filters will often flag such emails and stop delivery. It’s best to use a mix of text and images so that readers can understand your message whether images are turned on or off. If the message isn’t clear, recipients may merely hit the SPAM button or delete your message. If this happens, the deliverability of future emails will drop.

source Check Your HTML. Broken HTML code can lead to a poorly rendered message that generates complaints or prompt recipients to delete an unreadable message, causing future drops in deliverability.

Avoid base64. Spammers frequently use base64 encoding to hide their content from anti-spam software. Emails containing base64 are more likely to get flagged as SPAM by content filters.

Use Pre-Deployment Tools. Many email platforms include pre-deployment tools to scan your emails against multiple SPAM tests and identify issues that might keep you out of the inbox. They often provide actionable advice on how to fix these potential problems to improve deliverability before you hit the send button.

Additional Resources

The practice of email marketing is extremely complicated, yet many marketers are ill informed and approach it without the necessary knowledge or tools. The following posts will help you avoid common mistakes that not only affect the performance of current campaigns but also the deliverability of future campaigns.

For more in depth information, check out the following resources.



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