Summary
Email deliverability and spam filtering are complex issues influenced by multiple factors. While certain words and phrases like 'free', 'guarantee', and urgent calls to action can trigger filters, the context in which they are used and the sender's reputation play a significant role. Modern spam filters utilize machine learning and Bayesian filtering to analyze content patterns, and user complaints, misleading subject lines, high image-to-text ratios, suspicious attachments, and the absence of a clear unsubscribe option can all negatively impact deliverability. Maintaining a good sending reputation and adhering to email marketing best practices recommended by reputable ESPs are crucial for avoiding spam filters.
Key findings
- Common Spam Triggers: Words like 'free,' 'guarantee,' 'urgent,' and excessive use of capitalization or exclamation points can trigger spam filters.
- Context Matters: The context in which words are used, and the relationship between words, is increasingly important for spam filtering.
- Sender Reputation is Key: A sender's reputation is a primary factor in determining deliverability; a low reputation leads to aggressive filtering.
- Machine Learning & Bayesian Filtering: Modern spam filters use machine learning and Bayesian filtering techniques to identify spam patterns.
- User Engagement: High rates of user spam complaints directly and negatively impact email deliverability.
- Misleading Content: Misleading subject lines, fake deadlines, and content that doesn't match the subject line can lead to emails being flagged as spam.
- Technical Aspects: Technical aspects such as high image-to-text ratios, use of URL shorteners, and presence of suspicious attachments can trigger spam filters.
- Unsubscribe Option: Lack of a clear and easy unsubscribe option can result in emails being marked as spam.
Key considerations
- Monitor Sender Reputation: Actively monitor and maintain a positive sender reputation by following email marketing best practices.
- Be Mindful of Language: Use caution with common spam trigger words and ensure your language is clear, honest, and relevant.
- Provide Value to Subscribers: Focus on providing valuable content to subscribers to minimize spam complaints and encourage engagement.
- Test Email Content: Use spam filter checkers and testing tools to identify potential spam triggers before sending emails.
- Balance Image and Text: Maintain a balanced text-to-image ratio and avoid excessive use of images in your emails.
- Ensure Easy Unsubscribe: Provide a clear and easy-to-find unsubscribe option in every email you send.
- Follow ESP Guidelines: Adhere to the email marketing guidelines and best practices recommended by your Email Service Provider (ESP).
- Stay Updated: Keep up-to-date with the latest trends and techniques in email marketing and spam filtering to adapt your strategy accordingly.
What email marketers say
11 marketer opinions
Spam filters are triggered by a variety of factors beyond just specific words. While certain words like 'free' and 'guarantee' are known triggers, context, sender reputation, and email structure play crucial roles. Modern filters use machine learning to identify patterns in spam, and practices like misleading subject lines, excessive capitalization, high image-to-text ratio, suspicious attachments, and URL shorteners can also lead to emails being flagged as spam. Smaller providers may still rely on more basic, rule-based filtering systems.
Key opinions
- Word Triggers: Words like 'free,' 'guarantee,' and urgent terms can trigger filters, especially when combined with other red flags.
- Context Matters: Modern spam filters analyze the context and relationships between words, not just individual spam terms.
- Reputation: Sender reputation is a critical factor; a low reputation increases the likelihood of filtering, regardless of content.
- Misleading Content: Misleading subject lines, fake deadlines, and promises the email doesn't deliver can result in spam classification.
- Email Structure: A high image-to-text ratio and the use of URL shorteners can make emails appear suspicious.
- Attachments: Suspicious or uncommon attachments will cause the emails to get flagged.
- Machine Learning: Machine learning is now a major component for spam detection.
Key considerations
- Test Content: Regularly test email content with spam filter checkers to identify and remove potentially problematic elements.
- Monitor Reputation: Actively monitor and maintain a good sender reputation to improve deliverability.
- Be Transparent: Avoid misleading subject lines and ensure email content aligns with what is promised.
- Balance Content: Maintain a balanced text-to-image ratio and avoid excessive use of URL shorteners.
- Avoid Spammy Practices: Refrain from using excessive capitalization, exclamation points, or sales-oriented language.
- Keep Content Updated: Spam filtering evolves - keep your knowledge and best practices current.
Marketer view
Email marketer from Sendinblue shares that misleading subject lines, such as those promising something the email doesn't deliver, can lead to being flagged as spam.
22 Aug 2021 - Sendinblue
Marketer view
Email marketer from Moosend states that the overuse of sales-oriented language such as 'buy now,' 'best price,' or similar phrases can lead to emails being classified as spam.
5 Dec 2021 - Moosend
What the experts say
4 expert opinions
While certain words like 'free' and 'guaranteed' are commonly flagged as spam triggers, particularly when used excessively with exclamation marks or capitalization, modern spam filters prioritize context and sender reputation. A good sending reputation from a reputable ESP ensures better deliverability, even with potentially 'spammy' words. Conversely, senders with low reputation face aggressive filtering, irrespective of specific word choices.
Key opinions
- Common Spam Words: Words like 'free' and 'guaranteed,' especially with excessive capitalization or exclamation points, are common spam triggers.
- Context Matters: Modern spam filters analyze the context and relationships between words, not just individual spam words.
- Sender Reputation: Sender reputation is a crucial factor influencing deliverability; low reputation senders are aggressively filtered.
- Reputable ESPs: Following advice from a reputable ESP regarding avoiding spam filters can improve deliverability.
Key considerations
- Monitor Reputation: Actively monitor and maintain a positive sender reputation to improve email deliverability.
- Contextual Word Use: Be mindful of the context in which words are used, as it significantly impacts spam filtering decisions.
- Avoid Spammy Practices: Minimize excessive capitalization, exclamation points, and the use of known spam trigger words.
- Follow ESP Advice: Adhere to best practices recommended by reputable Email Service Providers to avoid spam filters.
Expert view
Expert from Email Geeks found advice on a reputable ESPs “how to avoid spam filters” page, linked to as “related content” from their abuse reporting form.
13 Jan 2025 - Email Geeks
Expert view
Expert from SpamResource explains that modern spam filters analyze the context and relationships between words, rather than just looking for individual 'spam words'. So, any word can be spammy depending on how it's used.
22 Nov 2022 - SpamResource
What the documentation says
4 technical articles
Spam filtering relies on various factors beyond specific words and phrases. SpamAssassin employs Bayesian filtering based on the frequency of terms in known spam. A missing or difficult unsubscribe option, as highlighted in RFC 2369, can flag emails. High user spam complaint rates, as monitored by Google Postmaster Tools, negatively affect deliverability. Finally, a low sender reputation, as noted by Microsoft, greatly increases the chances of emails being filtered as spam, regardless of content.
Key findings
- Bayesian Filtering: SpamAssassin uses Bayesian filtering, analyzing word frequencies in known spam to identify new spam.
- Unsubscribe Importance: The absence of a clear and easy unsubscribe option can lead to emails being flagged as spam.
- User Complaints: A high rate of user spam complaints directly and negatively impacts email deliverability.
- Sender Reputation: A low sender reputation greatly increases the likelihood of emails being filtered as spam.
Key considerations
- Easy Unsubscribe: Provide a clear and easily accessible unsubscribe option in all emails.
- Monitor Complaints: Actively monitor and address user spam complaints to maintain good deliverability.
- Protect Reputation: Take steps to protect and improve sender reputation, as it significantly affects filtering decisions.
- Content Analysis: Consider tools like SpamAssassin to analyze email content for potential spam triggers.
Technical article
Documentation from Google Postmaster Tools details that a high rate of user spam complaints directly impacts deliverability and can result in future emails being filtered as spam.
20 Apr 2024 - Google
Technical article
Documentation from Microsoft shares that a low sender reputation, due to past spam activity, will significantly increase the likelihood of emails being filtered as spam, irrespective of content.
23 Nov 2023 - Microsoft
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