Summary
Gmail's decision to mark emails as spam when it cannot verify the sender is multifaceted. Authentication failures (SPF, DKIM, DMARC) are primary indicators. Specific SPF issues include broken syntax, missing Google includes, and incorrect escaping. Beyond authentication, a poor IP/domain reputation from blocklisting or past spam activity significantly increases spam likelihood. Other factors include missing/incorrect reverse DNS records, spam trigger words, poorly formatted content, low engagement rates, absent unsubscribe links, inconsistent sending volume, lack of a dedicated/warmed-up IP, poor list hygiene and domain abuse. Google's anti-spam system combines technical validation, reputation assessment, and user engagement signals to safeguard users from unwanted mail.
Key findings
- Authentication Issues: Gmail marks emails as spam primarily when it can't verify the sender due to problems with SPF, DKIM, and DMARC.
- SPF Record Errors: Incorrect SPF syntax (e.g., missing Google includes, extra characters, incorrect escaping) leads to authentication failures.
- Reputation Problems: A poor IP or domain reputation, often due to blocklisting, increases spam classification.
- Engagement Impact: Low engagement (opens/clicks) and high spam complaint rates worsen sender reputation and trigger spam filters.
- Infrastructure Gaps: Missing reverse DNS, inconsistent sending volume, and lack of dedicated/warmed-up IPs contribute to spam classification.
- Content Triggers: Spam trigger words and poorly formatted HTML can cause emails to be flagged as spam.
- List Hygiene: Sending to unengaged users and a lack of unsubscribe links leads to more emails being marked as spam.
Key considerations
- Fix Authentication: Implement SPF, DKIM, and DMARC to verify sender identity.
- Correct SPF Records: Ensure SPF records have correct syntax, include necessary services, and escape special characters.
- Monitor Reputation: Regularly check IP and domain reputation on blocklists.
- Boost Engagement: Improve email engagement rates and make unsubscribing easy.
- Manage Infrastructure: Maintain consistent sending volume, set up reverse DNS, and warm up new IPs.
- Refine Content: Avoid spam trigger words and ensure properly formatted emails.
- Practice List Hygiene: Clean email lists to remove unengaged or invalid addresses.
What email marketers say
10 marketer opinions
Gmail marks emails as spam when it cannot verify the sender's identity, primarily due to issues with email authentication (SPF, DKIM, DMARC). Several other factors contribute, including a poor IP/domain reputation, missing or incorrect reverse DNS records, the use of spam trigger words, poorly formatted email content, low engagement rates, missing unsubscribe links, inconsistent sending volume, and the lack of a dedicated and warmed-up IP for high-volume senders. Gmail's spam filters are designed to protect users from phishing and unwanted emails, relying on a combination of technical checks, reputation assessments, and user engagement signals.
Key opinions
- Authentication: Missing or misconfigured SPF, DKIM, and DMARC records are primary reasons for Gmail marking emails as spam.
- Reputation: A poor IP or domain reputation due to blocklisting or previous spam activity significantly increases the likelihood of spam classification.
- Content: Use of spam trigger words and poorly formatted HTML content can trigger Gmail's spam filters.
- Engagement: Low engagement rates (opens, clicks) and high spam complaint rates negatively impact sender reputation and lead to spam filtering.
- Infrastructure: Missing reverse DNS records, inconsistent sending volume, and the lack of a dedicated, warmed-up IP address for high-volume senders contribute to spam classification.
Key considerations
- Implement Authentication: Ensure SPF, DKIM, and DMARC are correctly configured to verify sender identity.
- Monitor Reputation: Regularly check IP and domain reputation using blocklist monitoring services.
- Optimize Content: Avoid spam trigger words and ensure emails are well-formatted with both HTML and text versions.
- Encourage Engagement: Implement strategies to improve email engagement rates and provide easy unsubscribe options.
- Manage Infrastructure: Maintain consistent sending volume, configure reverse DNS records, and warm up dedicated IP addresses for high-volume sending.
Marketer view
Email marketer from SendPulse responds that low engagement rates (e.g., few opens and clicks) can negatively affect sender reputation over time. Gmail interprets low engagement as a sign that recipients don't want the emails, increasing the likelihood of spam classification.
2 Jul 2022 - SendPulse
Marketer view
Email marketer from SuperOffice suggests that if you're sending a large volume of emails, using a dedicated IP address and properly warming it up is crucial. Without a dedicated IP and a gradual warm-up, Gmail is more likely to see the email as unsolicited and filter it as spam.
15 Jul 2022 - SuperOffice
What the experts say
6 expert opinions
Gmail's inability to verify an email sender and subsequent spam classification largely stems from authentication failures (SPF, DKIM, DMARC), particularly issues with the SPF record itself. This includes broken or messy SPF records with missing components like the Google include, extra characters, or incorrect escaping. Beyond authentication, Gmail considers list hygiene, flagging emails sent to old or unengaged addresses as spam. Finally, the lack of proper email warming on new servers or IP addresses can also trigger spam filters, leading to emails being incorrectly classified.
Key opinions
- Authentication Failure: Issues with SPF, DKIM, and DMARC authentication are primary causes for Gmail's inability to verify senders.
- SPF Record Issues: Specific SPF record problems, such as broken syntax, missing Google includes, and incorrect escaping, contribute to authentication failure.
- List Hygiene: Sending emails to old or unengaged addresses negatively impacts sender reputation and increases spam classification.
- Lack of Email Warming: Failure to properly warm up new servers or IP addresses can lead to emails being incorrectly flagged as spam.
Key considerations
- Fix Authentication: Ensure correct implementation of SPF, DKIM, and DMARC to properly authenticate emails.
- Correct SPF Records: Carefully review and correct SPF records, ensuring proper syntax, inclusion of necessary services like Google, and correct escaping of special characters.
- Improve List Hygiene: Regularly clean email lists to remove old or unengaged addresses, reducing the risk of spam classification.
- Warm Up IPs: Implement a gradual warm-up process when introducing new servers or IP addresses, sending increasing volumes of emails over time.
Expert view
Expert from Spam Resource explains that Gmail often flags emails as spam when authentication checks, such as SPF, DKIM, and DMARC, fail. This is a primary indicator for Gmail that the sender may not be who they claim to be, leading to a higher likelihood of spam classification.
16 Feb 2023 - Spam Resource
Expert view
Expert from Word to the Wise explains that email warming is crucial when setting up new servers or IP Addresses to avoid spam filters. Sending emails from servers that haven't been warmed up often leads to emails being marked as spam.
2 Jan 2023 - Word to the Wise
What the documentation says
5 technical articles
Gmail's inability to verify a sender and subsequent spam classification primarily arises from improperly configured or failing email authentication methods, such as SPF and DKIM, as highlighted by Google Support, RFC-Editor, and DKIM.org. A valid DKIM signature is crucial to ensure the email's integrity, and its absence can lead to spam filtering. Additionally, being listed on real-time blocklists (RBLs) like Spamhaus signals a high risk of spam, causing Gmail to flag the email. Microsoft's documentation supports this, emphasizing the importance of correctly configured SPF, DKIM, and DMARC records, as well as avoiding any history of domain abuse to prevent spam classification.
Key findings
- Authentication Failure: Improperly configured SPF and DKIM are the main reasons Gmail cannot verify the sender.
- SPF Failures: SPF failures (softfail or fail) cause receiving mail servers to treat messages with suspicion, increasing the likelihood of spam classification.
- Missing DKIM Signature: Without a valid DKIM signature, Gmail lacks assurance that the email hasn't been altered, making it more prone to spam filtering.
- Blocklist Listing: If the sending server's IP is on a real-time blocklist (RBL), Gmail will likely mark the email as spam.
- Domain Abuse History: A history of domain abuse can lead to emails being classified as junk or spam.
Key considerations
- Implement Authentication: Ensure SPF and DKIM are properly set up to allow Gmail to verify the sender.
- Address SPF Failures: Investigate and resolve any SPF failures to reduce suspicion from receiving mail servers.
- Implement DKIM Signatures: Implement DKIM signatures to confirm the integrity of the email content.
- Monitor Blocklists: Regularly check if the sending server's IP is on any RBLs and take steps to be removed if listed.
- Avoid Domain Abuse: Ensure responsible email sending practices to avoid a history of domain abuse.
Technical article
Documentation from Microsoft explains that similar to Gmail, their email services also heavily rely on sender authentication methods. If SPF, DKIM, and DMARC records are not correctly configured, or if the domain has a history of abuse, emails are more likely to be classified as junk or spam.
12 Jul 2024 - Microsoft
Technical article
Documentation from Google Support explains that Gmail can't verify the sender when email authentication methods like SPF and DKIM aren't properly set up. This lack of verification can lead to Gmail marking the email as spam.
3 Feb 2022 - Google Support
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