How to get SMTP bounce logs from an ESP and what to do when emails go to spam?
Matthew Whittaker
Co-founder & CTO, Suped
Published 14 Aug 2025
Updated 19 Aug 2025
8 min read
Email deliverability is a complex landscape, and few issues are as frustrating as emails not reaching the inbox. When messages bounce back or, even worse, disappear into the spam folder, it's a clear signal that something is amiss. Understanding why this happens and how to retrieve crucial data, such as SMTP bounce logs from your Email Service Provider (ESP), is fundamental to diagnosing and resolving these problems.
Many email marketers face the challenge of deciphering email delivery issues, often relying on their ESPs for insight. However, obtaining detailed SMTP bounce logs can sometimes be an uphill battle. This guide will help clarify what bounce logs are, how you might be able to get them, and, crucially, what steps to take when your emails are consistently landing in the spam folder.
Understanding SMTP bounce logs
SMTP bounce logs are detailed records of why an email failed to deliver to a recipient's inbox. When an email bounces, the recipient's mail server typically sends back an SMTP (Simple Mail Transfer Protocol) error code and a descriptive message. These logs are invaluable because they provide specific reasons for non-delivery, ranging from temporary issues to permanent address problems.
Bounces are generally categorized into two main types: hard bounces and soft bounces. Knowing the difference is critical for maintaining a healthy email list and strong sender reputation.
Hard bounces: Indicate a permanent delivery failure. This could be due to an invalid email address (e.g., misspelled, non-existent) or a blocked domain. These addresses should be removed from your list immediately to prevent damage to your sender reputation. For more details on these types of failures, you can explore what a bounced email means.
Soft bounces: Are temporary delivery failures. Reasons include a full inbox, the recipient's server being temporarily down, or the message size exceeding limits. ESPs usually retry sending soft-bounced emails for a period. If the issue persists, a soft bounce can convert into a hard bounce. Learning how to troubleshoot high soft bounce rates can improve your deliverability.
Accessing ESP bounce data
Getting raw SMTP bounce logs directly from your ESP can be challenging. Many ESPs use third-party senders, meaning they might not have direct access to the most granular log data or are simply reluctant to provide it due to privacy concerns, the sheer volume of data, or the effort required to extract it. For example, some platforms, even when using sub-processors like Mailgun, might only show a bounce event in their dashboard without the full SMTP transcript.
However, most ESPs provide summary reports or dashboards that include bounce rates and general reasons for non-delivery. While not as granular as raw SMTP logs, these reports are often sufficient for identifying common issues. They should specify if a bounce was hard or soft, and sometimes even provide a brief description of the bounce reason. If you need more detail, submitting a specific, concise request to your ESP's support team can increase your chances of success. Asking for bounce data for a specific campaign or a small time frame is usually more feasible than asking for months of data.
For very specific debugging, some advanced ESPs might offer API access to event data, which can include detailed bounce information. Alternatively, if you manage your own mail server, you can access the SMTP logs directly, typically located in a directory like /var/log/mail.log on Linux systems. This level of access, however, is uncommon for most marketing teams using commercial ESPs.
Why emails go to spam
When emails go to the spam (or junk) folder, it's a different scenario than a bounce. A bounced email is rejected by the recipient server. An email sent to spam is typically *accepted* by the recipient server but then filtered into a secondary folder, meaning SMTP bounce logs often won't show a clear rejection. Instead, the logs will likely indicate that the email was accepted for delivery, even if it never reached the primary inbox.
Spam filtering is primarily determined by recipient mailbox providers (like Gmail, Yahoo, Outlook) based on a combination of factors related to sender reputation and content. To diagnose why your emails are landing in spam, you need to look beyond bounce logs. Tools like Google Postmaster Tools are invaluable here, as they provide insights into your domain and IP reputation directly from Gmail's perspective. Similarly, Postmark's guide offers further insights into this issue.
Common reasons emails get flagged as spam include low sender reputation, poor list quality (e.g., outdated or purchased lists), high spam complaint rates, lack of proper email authentication (SPF, DKIM, DMARC), and suspicious content (e.g., spammy keywords, broken links, image-heavy emails). If your email has been accepted by the server but still goes to spam, the problem lies within these areas, not necessarily with a bounce that generates a log.
Proactive measures for deliverability
To prevent emails from going to spam or bouncing, focusing on foundational deliverability practices is key. This includes maintaining a clean and engaged email list, implementing robust authentication protocols, and monitoring your sender reputation diligently.
Email authentication: Ensure your domain has correctly configured SPF, DKIM, and DMARC records. These protocols verify that your emails are legitimate and prevent spoofing. Missing or misconfigured records are significant red flags for spam filters. Using a free DMARC record generator can help you get started.
List hygiene: Regularly clean your email list by removing inactive subscribers and hard-bounced addresses. Sending to unengaged or invalid addresses signals poor sending practices to ISPs, leading to lower inbox placement. Aim for a healthy email bounce rate, ideally below 2% as suggested by Mailgun. Regular email testing can identify potential issues.
Content quality: Avoid spam trigger words, excessive use of all caps, and overly promotional language. Balance images with text and ensure your emails are well-formatted and responsive across devices.
Monitor sender reputation: Use tools like Google Postmaster Tools and your ESP's internal reporting to track your spam complaint rate and domain reputation. High complaint rates are a strong indicator of deliverability problems. Regularly check if your IP or domain is on any email blocklists (or blacklists).
By proactively managing these aspects, you can significantly improve your email deliverability and ensure your messages reach the intended audience, rather than bouncing or ending up in the spam folder.
Summary of key takeaways
Gaining access to detailed SMTP bounce logs from an ESP can be difficult, often due to the volume and complexity of the data. While these logs are crucial for understanding why emails bounce, they are typically less helpful for diagnosing why emails go to the spam folder, as such messages are usually accepted by the recipient server before being filtered. Instead, focus on proactive deliverability strategies like strong email authentication, maintaining a clean and engaged email list, and continuously monitoring your sender reputation and content quality. These actions will yield more consistent results in ensuring your emails reach the inbox.
Views from the trenches
Best practices
Always maintain a clean, opted-in email list to ensure better sender reputation and reduced bounce rates.
Implement SPF, DKIM, and DMARC for your sending domain to properly authenticate your emails.
Regularly monitor your domain and IP reputation using tools provided by mailbox providers.
Segment your audience and send relevant content to improve engagement and reduce spam complaints.
Common pitfalls
Requesting large volumes of raw SMTP logs from ESPs without specific parameters, which can be unfeasible.
Assuming that all deliverability issues are due to IP blacklists when content or recipient engagement is often the cause.
Neglecting email list hygiene, which leads to high bounce rates and potential spam trap hits.
Not understanding that emails landing in spam are often accepted by the server, making bounce logs unhelpful.
Expert tips
If emails are going to spam, look at address collection processes and prioritize engaged contacts.
For widespread deliverability issues, consider resting sends for a few days to reset engagement signals.
Gmail inboxing issues are rarely IP-related; focus instead on domain reputation and message content.
Clear and restricted data requests to ESPs have a higher chance of being fulfilled than broad ones.
Expert view
Expert from Email Geeks says SendGrid can provide log data, but it requires time and effort due to the volume and potential complexity of combining records.
2019-02-08 - Email Geeks
Expert view
Expert from Email Geeks says requesting 30 days of full bounce logs is excessive and often unnecessary for diagnosis, as it involves significant effort to process.
Gmail, 
Yahoo, Outlook) based on a combination of factors related to sender reputation and content. To diagnose why your emails are landing in spam, you need to look beyond bounce logs. Tools like Google Postmaster Tools are invaluable here, as they provide insights into your domain and IP reputation directly from Gmail's perspective. Similarly, Postmark's guide offers further insights into this issue.
