Summary
Despite SPF hard fails, emails can land in inboxes due to a multifaceted approach by email providers like Gmail and Microsoft. These providers consider more than just SPF, factoring in sender reputation, recipient engagement, content quality, and user behavior. A strong sender reputation, positive user engagement, and high-quality content can override SPF failures. Additionally, mailbox providers use machine learning and Spam Confidence Levels (SCL) to analyze emails, and DMARC policies dictate how SPF failures are handled. Spammers continually test filters, and factors like safe sender lists can influence delivery. In cases like forwarded mail, SPF checks might be unreliable, leading providers to rely on alternative signals.
Key findings
- Multi-Factor Authentication: Email providers use multiple factors beyond SPF, including sender reputation, engagement, and content.
- Reputation and Engagement Override: Strong sender reputation and positive user engagement can override SPF failures.
- Machine Learning Analysis: Email providers employ machine learning to analyze content and sending patterns.
- DMARC Policy: DMARC policy affects how SPF failures are handled, but enforcement varies.
- Spammer Adaptation: Spammers actively test and bypass filters.
- Alternative Signals: Forwarded mail relies on alternative signals due to potential SPF check failures.
Key considerations
- Build Sender Reputation: Focus on building a strong sender reputation.
- Encourage Engagement: Promote positive user engagement.
- High-Quality Content: Create high-quality, relevant content.
- Implement Authentication: Implement SPF, DKIM, and DMARC for authentication.
- Monitor DMARC: Monitor and adjust DMARC policy as needed.
- Adapt to Filters: Stay informed about evolving spam filters and adapt sending practices.
What email marketers say
12 marketer opinions
While an SPF hard fail indicates that an email should be rejected, Gmail and other mailbox providers use a complex algorithm to determine inbox placement. Several factors beyond SPF, such as sender reputation, recipient engagement, content quality, and user behavior, play a significant role. Positive engagement and a good sender reputation can override SPF failures, while negative factors can lead to spam placement, even with proper authentication. Machine learning and user feedback also influence Gmail's filtering decisions.
Key opinions
- Multi-Factor Analysis: Gmail uses a multi-factor analysis, including sender reputation, content, user engagement, and other signals, to determine inbox placement, rather than relying solely on SPF.
- Engagement Overrides SPF: Positive recipient engagement (opens, clicks) and a good sender reputation can override an SPF hard fail, leading to inbox placement.
- Machine Learning Influence: Gmail uses machine learning to analyze email content and sending patterns, further influencing filtering decisions beyond technical authentication.
- Spammer Techniques: Spammers are bypassing the traditional checks and balances.
Key considerations
- Sender Reputation: Building and maintaining a positive sender reputation is crucial for email deliverability, as it can outweigh authentication failures.
- Recipient Engagement: Encouraging positive recipient engagement (opens, clicks, replies) can improve inbox placement and reduce the impact of SPF failures.
- Email Content: Creating high-quality, relevant content can improve user engagement and signal trustworthiness to mailbox providers.
- Monitor Authentication: Monitoring and ensuring proper setup of authentication methods (SPF, DKIM, DMARC) is critical for increasing email deliverability.
- DMARC Policy: A DMARC policy that is set to none can override SPF records, and should be set to quarantine/reject.
Marketer view
Email marketer from Mailgun Support shares that although an SPF hard fail tells the receiving server the email should be rejected, the server may still choose to accept the email and deliver it to the inbox or spam folder. This is because the recipient server will take into account other factors and apply its own policies.
8 Mar 2022 - Mailgun
Marketer view
Email marketer from Email on Acid shares that recipient engagement is a major factor for getting emails into the inbox and that consistently good engagement can lead inbox providers to be more lenient regarding technical checks.
15 Jun 2023 - Email on Acid
What the experts say
3 expert opinions
Even when emails fail SPF checks, they might still land in the inbox instead of the spam folder due to several factors. Spammers are constantly testing filters to find ways to reach inboxes. Additionally, mailbox providers often rely on signals beyond SPF, such as sender reputation and recipient engagement history, particularly in cases like forwarded mail where SPF checks might be unreliable. A history of positive engagement from a recipient can lead providers to ignore SPF failures.
Key opinions
- Spammer Adaptation: Spammers actively test filters to find ways to bypass them and reach inboxes.
- Alternative Signals: Mailbox providers consider sender reputation and recipient engagement when SPF checks fail.
- Engagement History: A history of positive engagement can cause mailbox providers to overlook SPF failures.
Key considerations
- Sender Reputation: Building and maintaining a good sender reputation is crucial, as it serves as a key signal for inbox providers.
- Recipient Engagement: Encouraging positive recipient engagement can improve inbox placement, even when SPF checks fail.
- Spam Filter Evolution: Spam filters are dynamic, so senders should continually monitor and adjust their sending practices to stay ahead of spammers.
Expert view
Expert from Spam Resource explains that in some cases, especially with forwarded mail, SPF checks can fail, and receivers must rely on other signals (like reputation) to determine deliverability.
25 Nov 2022 - Spam Resource
Expert view
Expert from Spam Resource explains that mailbox providers may choose to ignore SPF failures and rely on other signals, especially when there's a history of positive engagement from the recipient.
31 May 2023 - Spam Resource
What the documentation says
4 technical articles
Although SPF is a key email authentication method, it's just one factor considered by email providers like Gmail and Microsoft. Email providers use a combination of factors, including sender reputation, content analysis, user behavior, and DMARC policy, to determine whether an email lands in the inbox, spam folder, or is rejected. DMARC policy influences how receiving servers handle SPF failures, but even a 'reject' policy may not always be strictly enforced. Ultimately, the receiving server has the authority to decide how to handle messages, and factors like safe sender lists can override SPF failures.
Key findings
- SPF is One Factor: SPF is an important email authentication method, but it's only one of many factors used by email providers.
- DMARC Policy Influences Handling: DMARC policy dictates how receiving servers should handle messages that fail SPF checks, but enforcement isn't always strict.
- Receiver Authority: The receiving server ultimately decides how to handle messages, and other factors can override SPF failures.
- SCL Score: Microsoft uses a Spam Confidence Level (SCL) score that considers multiple factors of which SPF is one.
Key considerations
- Comprehensive Authentication: Implement SPF, DKIM, and DMARC to improve email authentication and signal trustworthiness to email providers.
- Monitor DMARC Policy: Regularly monitor your DMARC policy and adjust it as needed to ensure it aligns with your email sending practices.
- Build Sender Reputation: Focus on building a positive sender reputation by following email best practices and engaging with recipients.
- Consider User Signals: Be aware that user behavior, such as marking emails as 'not spam,' can override technical authentication checks.
Technical article
Documentation from DMARC.org shares that while SPF is an important authentication method, DMARC policy dictates how receiving mail servers should handle messages that fail SPF checks. A policy of 'none' allows messages to be delivered regardless of SPF results, and even a 'reject' policy may not be strictly enforced by all mail servers.
4 Jul 2023 - DMARC.org
Technical article
Documentation from RFC Editor explains that SPF provides a mechanism for verifying the sender's authorization to use a domain but that the receiving server has the ultimate authority on how to handle messages that fail SPF checks. It recommends policies, but doesn't mandate actions.
2 Dec 2021 - RFC Editor
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