Why do SendGrid transactional emails go to spam but campaign emails go to inbox?

Key findings

• Content differences: Even with identical sending infrastructure, variations in content, headers, or even the underlying code (API vs. campaign tool) can trigger spam filters. Filters analyze thousands of data points beyond just the sender and IP.
• Sending behavior and reputation: Sending too many test emails without engagement can negatively train your own inbox or the mailbox provider about your transactional email stream. Mailbox providers monitor recipient engagement and how emails are handled, which impacts sender reputation.
• Authentication variations: SPF, DKIM, and DMARC, between transactional and campaign sends could lead to different deliverability outcomes. Even minor misconfigurations can cause issues.
• Mailbox provider fingerprinting: Mailbox providers (like Gmail) are adept at fingerprinting different sending patterns, even from the same source. This can include subtle variations in headers, cadence, or even payload size, which can affect where messages land.
• Segregation of mail streams: While shared IPs are common on platforms like SendGrid, some best practices recommend separating transactional and marketing email streams on different IPs or even subdomains to isolate reputation issues. This is because marketing emails often have higher spam complaint rates.

Key considerations

• Analyze full email headers: Compare the full email headers of a transactional email that went to spam versus a campaign email that went to the inbox. Look for subtle differences in headers added by SendGrid or your application, or any authentication discrepancies.
• Review content thoroughly: Even if the visible content seems similar, underlying HTML, links, image ratios, or hidden characters can impact spam scoring. Use a spam checker to evaluate both types of emails.
• Monitor engagement for test sends: When testing, ensure you actively open and interact with the transactional emails to build positive signals with the mailbox provider. Repeatedly sending tests that are ignored or deleted can damage your personal inbox's perception of those emails.
• Check SendGrid's performance metrics: Utilize SendGrid's analytics to compare the performance metrics (opens, clicks, spam reports, bounces) of your transactional emails versus your campaign emails. This can highlight where deliverability issues are truly occurring.
• Consult SendGrid's deliverability best practices: Refer to official documentation for best practices, especially regarding the nuances of different email types and their impact on deliverability. SendGrid themselves advise against mixing certain types of mail, as seen in their support article on email deliverability.

Key opinions

• Content is king: Many marketers believe that even slight differences in the content of transactional emails versus campaign emails can be the primary reason for varied inbox placement. Spam filters are sophisticated and analyze thousands of content data points.
• Self-inflicted damage: A common theory is that repeatedly sending transactional test emails to personal inboxes, then deleting them without engagement, can train the individual mailbox or the provider to classify these specific sends as spam.
• Mailbox provider differences: The issue might be specific to certain mailbox providers, such as Gmail, which are known for their advanced fingerprinting capabilities that detect subtle differences in sending patterns and content, as detailed in our guide on troubleshooting transactional emails going to spam at Gmail.
• API vs. campaign tool: While the underlying data might be similar, the method of sending (e.g., via API versus a campaign interface) can introduce differences in headers or other meta-data that influence deliverability.
• Impact of poor list hygiene: Sending transactional emails to inactive or unsubscribed users is a significant risk factor that can lead to spam folder placement, even for typically high-deliverability messages.

Key considerations

• Deep content analysis: Conduct a meticulous comparison of the content, including HTML structure, link tracking, and any hidden elements, between the problematic transactional emails and successful campaign emails.
• Review test email engagement: Ensure that test emails are opened and interacted with to avoid inadvertently training spam filters against your own sends. Consider using a dedicated seed list or testing approach.
• Check specific ISP behavior: Identify if the issue is universal or specific to certain mailbox providers, as their filtering algorithms differ. This could indicate a need for targeted adjustments.
• Analyze performance data: Leverage SendGrid's reporting features to review detailed performance metrics for both email types. Look for discrepancies in open rates, click-through rates, and complaint rates that could signal underlying issues. To proactively identify such issues, consider using an email deliverability tester.
• Consider email type segregation: If problems persist, evaluate the benefits of separating transactional and marketing email streams onto distinct IPs or subdomains within SendGrid, as mixing these types can increase spam report rates for transactional mail.

Key opinions

• Authentication is paramount: Experts consistently stress the importance of robust and correctly configured email authentication protocols (SPF, DKIM, DMARC). Even if a domain is shared, discrepancies in how these are applied to different mail streams can lead to deliverability issues for specific email types.
• Header and content analysis: A deep dive into email headers and content is crucial. Experts recommend comparing the full, raw email source of a spam-filtered transactional email with a successfully delivered campaign email to identify subtle, yet critical, differences.
• ISP-specific filtering: Mailbox providers employ sophisticated algorithms, and their fingerprinting capabilities mean they can distinguish between transactional and marketing sends even from the same IP. This behavior is particularly noticeable with large providers like Gmail, as explored in our guide on email deliverability issues.
• Cadence and volume impact: The pattern and volume of sends, especially for test emails, can influence deliverability. Irregular or excessive testing without positive engagement can inadvertently harm reputation for those specific email types.
• Reputation segmentation: Some experts advocate for separating transactional and marketing email streams, often using different subdomains or IPs, to ensure that the good reputation of transactional emails is not negatively impacted by the typically lower engagement or higher complaint rates of marketing emails.

Key considerations

• Detailed header comparison: Use tools to compare the full, parsed headers of both email types. Look for unexpected additions, removals, or modifications, particularly related to authentication stamps like DKIM signatures or SPF alignment.
• Content variations, visible and invisible: Beyond the visible text, examine the HTML, CSS, image paths, and tracking pixels for any elements that might be interpreted differently by spam filters in a transactional context versus a marketing one. Even small changes can affect spam scoring.
• DMARC and SPF alignment: Verify that DMARC and SPF alignment are consistently passing for both email types. A failing alignment on transactional emails could be a critical indicator, even if campaigns pass. Our DMARC record generator can help check configurations.
• Subdomain strategy: Consider if using separate subdomains for transactional and marketing mail (e.g., trans.yourdomain.com vs. marketing.yourdomain.com) would provide better reputation isolation and more predictable deliverability for each stream. This is a common strategy to mitigate risk.
• Engagement measurement: Ensure that your transactional emails are designed to encourage positive engagement (opens, clicks) and minimize negative actions (spam complaints, deletions). High engagement signals trust to mailbox providers.

Key findings

• Sender reputation is key: Documentation from major email providers consistently states that a strong sender reputation is the paramount factor for inbox delivery. This reputation is an assessment by mailbox providers of your trustworthiness as a sender.
• Separation of mail streams: Many providers, including SendGrid, strongly advise against mixing marketing and transactional emails on the same sending IP. Marketing emails typically have higher spam complaint rates, which can negatively impact the deliverability of transactional mail on shared IPs.
• Engagement metrics matter: Documentation highlights that recipient engagement (opens, clicks, replies) directly influences deliverability. Low engagement, or negative actions like marking as spam, signals to providers that recipients do not value the mail, leading to spam folder placement or rejection.
• Transactional emails are distinct: Transactional emails are defined as direct responses to user actions (e.g., order confirmations, password resets). They are expected to be timely and highly relevant, often giving them a deliverability advantage over marketing messages, provided other factors are in order.
• Authentication is fundamental: Official guides reinforce that proper email authentication (SPF, DKIM, DMARC) is a non-negotiable prerequisite for good deliverability, ensuring that your emails are verified as legitimate and from the claimed sender.

Key considerations

• Adhere to best practices: Rigorously follow the email deliverability best practices outlined by your ESP (like SendGrid) and major mailbox providers. This includes list hygiene, managing bounce rates, and monitoring spam complaints.
• Utilize dedicated IPs/subdomains: If mixing mail types is causing issues, consider establishing dedicated IPs or subdomains for transactional emails to build and maintain a distinct, high-quality reputation for these critical messages.
• Prioritize authentication configuration: Regularly verify that your SPF, DKIM, and DMARC records are correctly configured and aligned for both your domain and any subdomains used for sending. This is a foundational element of email trust, as highlighted in guides to DMARC, SPF, and DKIM.
• Monitor deliverability metrics: Use the analytics and reporting tools provided by your ESP to closely track deliverability rates, open rates, click rates, and especially spam complaint rates for both transactional and marketing email streams.
• Content relevance: Ensure that the content of transactional emails remains strictly relevant to the user's action and does not contain promotional elements that could lead to it being classified as marketing mail by filters.