Why is your newsletter being flagged as spam?

Few things are as frustrating as sending a high-quality newsletter only for it to go straight to the recipients' spam folder.

Anyone who creates newsletters knows that valuable content takes time to produce. First you identify the topic that delivers maximum value to the recipient, then you ensure everything is correct—verifying that all links work, checking that the layout is responsive on both mobile and desktop, and proofreading the copy. Once complete, you hit send. Shortly after, you receive feedback that it landed directly in the spam folder.

What is spam?

Spam, or junk mail, is defined by Wikipedia as unsolicited bulk messages sent on the internet, and is most commonly associated with unsolicited email advertising.

Most people view spam as marketing emails received without prior consent. In Norway, sending unsolicited marketing emails is prohibited under Section 15 of the Marketing Control Act.

... and why does your newsletter end up there?

The market is filled with various spam filters, each with different rules for classifying emails. When a spam filter scans your newsletter, it applies these rules; if your email triggers one or more of them, it receives spam points. The higher the score, the more likely the email is to be flagged and filtered into the junk folder.

Because every spam filter operates independently and sets its own threshold for flagging emails, a newsletter that successfully passes through one filter might be blocked by another.

How spam filters operate

The criteria for assigning spam points evolve constantly. Filter databases are regularly updated based on what users mark as spam in their inboxes, and filters synchronize with each other to share new rules.

While there is no definitive master list of these rules, you can significantly reduce your spam score by following these guidelines:

• Avoid sending newsletters that consist of a single large image with little to no text (aim for a text-to-image ratio between 60/40 and 80/20).

• Do not use spam-trigger words like "Free", "Click here!", or "Save 70%".

• Avoid using dates in the subject line.

• Avoid using symbols that resemble spam, such as !! #$%&.

• Using the word "Test" in the subject line often triggers spam points.

• Do not use excessively large font sizes.

• Never use all caps in the body text or the subject line.

• Keep subject lines concise (ideally between 35 and 50 characters).

• Limit the number of different text colors.

• Ensure there is sufficient contrast between the text color and the background.

• Always check that no placeholder text ("Lorem ipsum...") remains in the email.

• If the offer in your newsletter sounds too good to be true, recipients are highly likely to mark it as spam.

• Avoid using free webmail addresses like Gmail or Hotmail as your sender address.

• Use a professional sender address associated with your website domain.

• Ensure your website is active and fully functional.

• Never buy, rent, or copy third-party email lists.

• Build your own contact list using opt-in forms on your website. Double opt-in is highly recommended.

• Bounced emails increase your spam score. Repeatedly sending to addresses that bounce will damage your overall deliverability.

When using an email marketing platform, it is critical to verify your sender domain so the system can send authorized emails on your behalf. This is configured using DKIM and SPF protocols. Following these best practices helps your domain establish a strong sender reputation, ensuring higher delivery rates.

Spam testing

Even if you follow these guidelines perfectly, your emails may still trigger some spam filters. This is common, as filters are so complex that avoiding every single rule is almost impossible. In fact, research shows that 70 percent of all emails contain at least one spam-related issue that can impact deliverability.

This makes running spam tests essential before sending. A spam test analyzes whether your email is likely to be flagged, checking your content against current filter rules and assessing your domain reputation.

While manual reviews are helpful and improve with experience, automated tests detect technical issues that are invisible to the naked eye. Testing your newsletter provides the necessary data to resolve hidden issues before they affect your campaign performance.