How to tell if soft bounces were eventually delivered?
Michael Ko
Co-founder & CEO, Suped
Published 25 Apr 2025
Updated 19 Aug 2025
8 min read
Email marketing is a cornerstone of digital communication, but sometimes your messages don't reach their intended recipients on the first try. One common scenario is a soft bounce, which indicates a temporary delivery issue. Unlike hard bounces, which signify permanent failures, soft bounces suggest that an email might eventually be delivered. The crucial question then becomes, how can you determine if those temporarily undeliverable messages eventually made it to the inbox? This is a challenge because the initial bounce notification doesn't confirm future delivery.
Understanding if a soft bounce transforms into a successful delivery is vital for accurate campaign performance measurement and maintaining a healthy email list. It impacts how you manage your contacts, whether you re-engage them, or if you need to investigate deeper deliverability issues. Without this insight, you might incorrectly categorize recipients, leading to missed opportunities or unnecessary list cleaning.
Let's explore the methods and data points available to track the journey of a soft-bounced email, from its initial hiccup to potential inbox arrival. We'll delve into examining logs, understanding different bounce classifications, and leveraging various reporting metrics to gain clarity on your email deliverability.
Understanding soft bounces and delivery attempts
Soft bounces are transient email delivery failures, meaning the recipient's mail server temporarily rejected the email. Common reasons include a full inbox, server downtime, or the message being too large. When this happens, your Email Service Provider (ESP) typically attempts to resend the email several times over a period, ranging from hours to days. The goal is for the receiving server to eventually accept the message once the temporary issue is resolved.
The ambiguity arises because different ESPs may classify and report these temporary failures differently. Some might immediately label a delayed email as a soft bounce, while others might consider it a 'deferred' or 'transient' error, only classifying it as a soft bounce if all retry attempts fail. This can lead to confusion when trying to ascertain true delivery status, as a message might still be in a retry queue without being explicitly reported as a soft bounce in your dashboard.
To effectively tell if soft bounces were eventually delivered, you need to understand your ESP's specific bounce handling logic and the level of detail they provide in their logging and reporting. This often involves looking beyond summary statistics and diving into raw data. This is crucial for optimizing your email deliverability strategy and maintaining a clean sender reputation.
Here are some common SMTP (Simple Mail Transfer Protocol) response codes often associated with soft bounces. These codes indicate temporary issues, and an ESP will typically retry sending the email.
SMTP Code
Meaning
Common Cause
421
Service not available, closing transmission channel
Recipient's server is busy or temporarily unavailable.
450
Requested mail action not taken: mailbox unavailable
Mailbox is temporarily full or unreachable.
451
Requested action aborted: local error in processing
Server-side error, often temporary, preventing immediate delivery.
452
Requested action not taken: insufficient system storage
Recipient's mailbox is full.
Leveraging your ESP's data and logs
The most reliable way to know if a soft-bounced email was eventually delivered is to check your ESP's detailed logs or delivery reports. Most reputable ESPs maintain comprehensive logs that record every attempt, failure, and eventual success for each email sent. You need to look for a 'delivered' event that corresponds to the message that initially soft bounced.
When an email is sent, it's typically assigned a unique message ID. When a soft bounce occurs, this ID will be associated with the bounce event. If the email is successfully delivered after retries, the same message ID (or a related one) should appear in a 'delivered' event within your logs. Many ESPs also use VERP (Variable Envelope Return Path), where unique return-path mailboxes are used, making cross-referencing easier. This ensures you can tie a specific bounce event to a later delivery status.
If your ESP's user interface doesn't readily expose this granular data, it's worth reaching out to their support team. They can often provide access to more detailed logs or explain how to interpret the available metrics to determine if a soft bounce later became a delivery. Many platforms will explicitly show a status change from 'deferred' or 'soft bounced' to 'delivered' for an individual recipient.
Example log entry for deferred and delivered status
Beyond server logs: secondary indicators of delivery
While server logs provide the most accurate picture, secondary indicators such as open and click tracking can offer insights into whether an email that initially soft bounced was eventually delivered. If a recipient opens or clicks a link in an email that was previously marked as a soft bounce, it's a strong indication that the message did, in fact, reach their inbox after a retry.
However, it's important to approach these metrics with caution. Open tracking, especially, has become less reliable due to privacy features like Apple Mail Privacy Protection. These features can artificially inflate open rates, making it appear as though an email was opened even if it wasn't by the human recipient. Click tracking remains a more dependable indicator, as it requires direct user interaction with a link within the email. Still, not all delivered emails are clicked or opened.
Therefore, while engagement metrics can serve as supplementary evidence, they should not be the sole basis for determining whether a soft-bounced email was eventually delivered. Always prioritize direct delivery status reports from your ESP's logs for the most accurate information. These metrics are more useful for understanding overall email performance and audience engagement.
Server-side tracking (logs)
This method provides definitive proof of delivery. Your ESP's mail servers communicate directly with recipient servers, recording precise delivery statuses, including successful retries after an initial soft bounce. It's the most reliable source for confirming if an email eventually reached its destination.
Accuracy: Highest accuracy in determining actual delivery to the mailbox.
Data source: ESP mail logs, SMTP response codes.
Drawbacks: May require technical expertise to access and interpret raw logs.
Client-side tracking (opens and clicks)
Engagement metrics like opens and clicks can indicate that a previously soft-bounced email was delivered, as interaction only occurs post-delivery. However, these are indirect measures and are subject to limitations, particularly with modern privacy features that can skew open rates.
Accuracy: Lower accuracy, especially for opens.
Data source: ESP analytics dashboards for campaign performance.
Drawbacks: Not all delivered emails are engaged with, and open rates can be unreliable.
Strategies for improving deliverability and reducing soft bounces
To minimize the occurrence of soft bounces and improve your overall deliverability, it's essential to adopt best practices for your email campaigns. Proactive management of your email list and sending habits can significantly reduce temporary delivery issues. A high soft bounce rate (or blocklist rate) can negatively affect your sender reputation and inbox placement, even if emails eventually deliver.
One key strategy is regular list hygiene. This means routinely removing inactive or problematic email addresses that consistently generate soft bounces. If an email address continually soft bounces over multiple campaigns, it may indicate a deeper issue that won't resolve on its own, such as a dormant or abandoned mailbox. Implementing a suppression logic after a certain number of soft bounces is a recommended approach. You can refer to this Mailchimp guide on soft bounces and hard bounces for more insights.
Additionally, optimizing your email content, ensuring proper authentication (like SPF, DKIM, and DMARC), and managing your sending volume can reduce the likelihood of encountering temporary server issues. Large attachments or overly complex HTML can trigger size-related soft bounces, while sudden spikes in sending volume can lead to throttling by recipient servers. Monitoring your email deliverability regularly can help you catch and address these issues promptly.
Regularly cross-reference your ESP's delivery logs with any soft bounce reports to confirm if the messages were eventually delivered.
Implement a clear bounce management policy to suppress addresses that consistently soft bounce after multiple retries, protecting your sender reputation.
Monitor bounce rates for trends, as sudden spikes can indicate larger underlying issues with your sending practices or recipient servers.
Communicate with your ESP about the granularity of their bounce data and whether they provide direct insights into soft bounce retry successes.
Common pitfalls
Mistaking a 'deferred' status for a permanent soft bounce, leading to premature suppression of valid email addresses.
Relying solely on open and click rates to confirm delivery, especially with email privacy features affecting tracking accuracy.
Failing to segment lists based on bounce history, continuing to send to problematic addresses and harming overall deliverability.
Not understanding the difference in how various ESPs classify and report temporary delivery failures and delays.
Expert tips
Utilize VERP (Variable Envelope Return Path) if your system supports it, as it simplifies the process of tying individual bounce notifications to specific email sends.
Look for email headers or SMTP response codes in your logs that indicate a temporary rejection, then track the message ID for subsequent delivery.
A soft bounce doesn't always mean failure; many systems will retry sending for up to 72 hours before classifying it as a final bounce.
Consider the age of the email address; older, less active addresses are more prone to soft bounces like mailbox full errors.
Expert view
Expert from Email Geeks says that you should look for a delivered event in your logs that matches the soft bounce. Having a way of cross-referencing in your own logs, particularly with VERP, makes this easiest.
2020-01-03 - Email Geeks
Marketer view
Marketer from Email Geeks says that any message that soft bounces will not be re-attempted to be delivered, or if it is, it is only with a new campaign. This depends on the ESP.
2020-01-03 - Email Geeks
Final thoughts on soft bounce delivery
Determining if soft bounces were eventually delivered is crucial for refining your email strategy and maintaining a high level of email deliverability. While the ambiguity in how different ESPs classify temporary failures can be challenging, the most reliable approach involves diving into your ESP's detailed logs and tracking specific message IDs or return paths. These logs provide the authoritative record of an email's journey from send to eventual delivery or final bounce.
Beyond log analysis, secondary indicators like engagement metrics (opens and clicks) can offer supporting evidence, though they should be used with an understanding of their limitations. Proactive measures, such as vigilant list hygiene and careful sending practices, are also paramount to minimize soft bounces from occurring in the first place. By combining robust data analysis with preventative strategies, you can ensure your messages reach their intended audience more consistently.
Apple Mail Privacy Protection. These features can artificially inflate open rates, making it appear as though an email was opened even if it wasn't by the human recipient. Click tracking remains a more dependable indicator, as it requires direct user interaction with a link within the email. Still, not all delivered emails are clicked or opened.
