Why am I getting soft bounces and how do I fix them?
Michael Ko
Co-founder & CEO, Suped
Published 27 Apr 2025
Updated 16 Aug 2025
6 min read
When you send an email, you expect it to land in the recipient's inbox. However, sometimes emails don't reach their destination and bounce back. These bounces can be frustrating, especially when they prevent your important messages from being delivered. Among the different types of bounces, soft bounces are particularly common and often misunderstood.
A soft bounce indicates a temporary email delivery issue. Unlike hard bounces, which signify a permanent problem (like an invalid email address), soft bounces suggest that the email address is valid but the message couldn't be delivered for a transient reason. If a soft bounce occurs, email servers typically attempt to resend the email for a certain period, hoping the issue resolves itself. However, if the temporary problem persists over multiple attempts, a soft bounce can eventually turn into a hard bounce.
Understanding why your emails are soft bouncing and how to address these issues is essential for maintaining a healthy sender reputation and ensuring your messages reach their intended recipients. I'll explain the common causes of soft bounces and provide practical steps to fix them, helping you improve your overall email deliverability.
Understanding soft bounces and their impact
Soft bounces can stem from various temporary issues, often related to the recipient's mail server or mailbox status. Recognizing the specific cause is the first step toward effective troubleshooting.
One of the most frequent reasons is a full mailbox. If a recipient's inbox has reached its storage limit, new incoming emails will be temporarily rejected, leading to a soft bounce. This is a common occurrence, particularly with free email providers where storage limits might be tighter or less actively managed by the user. Another common cause is a recipient's server being temporarily unavailable or offline. This could be due to maintenance, unexpected outages, or high traffic volume overwhelming the server. In such cases, the server cannot accept incoming mail at that moment, resulting in a soft bounce code.
Beyond server availability and mailbox capacity, soft bounces can also occur if the email message itself is too large, exceeding the recipient's server's size limits. Additionally, overly strict spam filters or blocklists (sometimes called blacklists) on the recipient's side might temporarily reject an email if its content, sender IP, or domain reputation is deemed suspicious. It is important to review email blocklists regularly.
Typical causes
Full mailbox: The recipient's inbox has exceeded its storage quota.
Server unavailable: The recipient's mail server is temporarily down or too busy.
Message too large: The email exceeds the size limit set by the recipient's server.
Content issues: The email content triggers spam filters.
Domain or IP issues: Your sending domain or IP address is temporarily blocklisted.
Diagnosing soft bounce issues
To effectively fix soft bounces, you first need to understand the exact reason behind them. The most crucial step is to retrieve and analyze the specific bounce messages or error codes returned by the recipient's mail server. Your email service provider (ESP) should provide access to these detailed bounce logs.
These messages often contain numerical codes (like 4xx errors for temporary failures) and human-readable explanations that pinpoint the issue. For instance, you might see messages indicating mailbox full, server busy, or message size exceeded. Analyzing these codes and descriptions is key to formulating a targeted solution.
Common SMTP Soft Bounce Codesplaintext
SMTP Error 421: Service not available, closing transmission channel
SMTP Error 450: Requested mail action not taken: mailbox unavailable
SMTP Error 451: Requested action aborted: local error in processing
SMTP Error 452: Requested action not taken: insufficient system storage
Once you have the bounce messages, look for patterns. Are the soft bounces primarily coming from a specific domain (e.g., Yahoo or Gmail)? Is there a sudden spike in soft bounces after a particular campaign or a change in your sending infrastructure? Identifying these trends will help you narrow down the potential culprits and troubleshoot the root cause more efficiently. Analyzing the specific context, like sending frequency or how old the addresses are, is also important.
Strategies to reduce soft bounces
Once you've diagnosed the specific reasons for your soft bounces, you can implement targeted strategies to mitigate them and improve your email deliverability. A primary approach involves maintaining excellent list hygiene. Regularly cleaning your email list helps remove inactive or problematic addresses that might be contributing to bounces.
For issues related to full mailboxes or busy servers, your ESP will usually retry sending the email automatically. However, for consistent issues, consider segmenting your audience and adjusting sending volume or frequency to specific domains that are causing trouble. This can help alleviate pressure on receiving servers and reduce the likelihood of your emails being throttled or temporarily rejected. Also, reviewing your email content for potential spam triggers is a good practice. High-quality, relevant content that avoids suspicious phrases or excessive links can help bypass aggressive spam filters. Refer to Gmail's guidelines for more insight.
Identifying problem areas
Monitor bounce codes: Regularly check your ESP's bounce logs for detailed error messages.
Analyze patterns: Determine if bounces are concentrated at specific domains or during certain campaigns.
Implementing solutions
Clean your list: Remove inactive or unengaged subscribers from your mailing list.
Adjust sending: Reduce volume or frequency to domains experiencing high soft bounces.
Optimize content: Ensure your emails are engaging and free of spammy characteristics.
Additionally, regularly monitoring your sender reputation and blocklist status (or blacklist status) can preemptively address issues that might lead to soft bounces. A sudden increase in soft bounces could indicate a developing reputation problem, even if it's not yet a hard bounce. Being proactive about these temporary failures helps prevent them from escalating into more serious deliverability challenges that could impact your overall email program.
Views from the trenches
Best practices
Maintain rigorous email list hygiene by regularly removing inactive or unengaged subscribers.
Analyze bounce messages and codes from your ESP to pinpoint specific reasons for soft bounces, such as full mailboxes or server issues.
Monitor your sender reputation and blocklist status to proactively address potential deliverability problems.
Common pitfalls
Ignoring soft bounce messages, as they can escalate into hard bounces and negatively impact sender reputation.
Sending emails with excessively large attachments or oversized content that exceeds recipient server limits.
Failing to segment lists and adjust sending patterns for domains that consistently generate soft bounces.
Expert tips
Implement a consistent sending schedule to build a stable sender reputation with mailbox providers.
Regularly test email deliverability using an email testing tool before sending large campaigns.
Engage in two-way communication with subscribers to confirm engagement and prevent list decay.
Marketer view
Marketer from Email Geeks says understanding the classification and originating provider of soft bounces is crucial as a first step in diagnosis.
2023-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks says reviewing the actual bounce messages from the sending server is the only reliable way to diagnose temporary email delivery failures.
2023-05-15 - Email Geeks
Final thoughts on managing soft bounces
Soft bounces are a common part of email sending, but they don't have to significantly impact your deliverability. By actively monitoring bounce messages, understanding their underlying causes, and implementing proactive strategies, you can manage and reduce soft bounce rates. This attention to detail ensures your emails consistently reach their recipients, strengthening your sender reputation and the effectiveness of your email campaigns.
