How can I reduce soft bounces after a one-day email volume spike?
Michael Ko
Co-founder & CEO, Suped
Published 20 Jul 2025
Updated 15 Aug 2025
8 min read
Experiencing a sudden spike in soft bounces after a one-day email volume surge can be alarming. It indicates that mailbox providers are temporarily deferring your emails, often due to an unexpected change in your sending patterns. While soft bounces aren't as severe as hard bounces, a sustained high rate suggests that your sender reputation is under strain.
Mailbox providers, such as Google and Yahoo, closely monitor sending behavior. A drastic, sudden increase in volume, even if the recipients are engaged, can trigger their spam filters or throttling mechanisms. This isn't necessarily a direct blocklisting, but rather a cautionary measure by the ISP to protect their network and users from potential spam or abuse. Effectively managing and reducing these soft bounces requires a multi-faceted approach, focusing on understanding the underlying causes and implementing strategic adjustments to your sending practices and email list hygiene.
How volume spikes affect sender reputation
A sudden, one-day spike in email volume sends a strong signal to Internet Service Providers (ISPs) that something unusual is happening with your sending domain or IP address. Even if the content and recipients are legitimate, this abrupt change deviates from your typical sending patterns, which can be interpreted as suspicious behavior. ISPs prefer consistent, predictable sending volumes, as this indicates a stable and trustworthy sender. When this pattern is broken, particularly with a significant increase, it can lead to immediate reputation downgrades.
These reputation hits often manifest as increased soft bounces, or deferrals, rather than outright hard bounces or direct spam folder placement. ISPs might choose to throttle your mail stream, meaning they accept a limited number of your emails per hour or day, and defer the rest. This deprioritization can also be a precursor to more severe issues, such as being placed on internal (private) blocklists or having your emails routed directly to the spam folder. Understanding this behavior is crucial for effective deliverability management.
While you might not immediately see a drop in inbox placement to the spam folder, a persistent 12% soft bounce rate after a spike is a clear indication that your sender reputation is grey enough to warrant closer monitoring by mailbox providers. This reputation hit can linger, affecting subsequent campaigns even if you return to normal volume. It's a signal that ISPs want you to scale back and demonstrate consistent, positive engagement.
Identifying the source of soft bounces
The first critical step in addressing high soft bounces is to identify exactly which Internet Service Providers (ISPs) are generating these deferrals. Not all ISPs will react to a volume spike in the same way. Some might be more sensitive to changes in sending patterns or have stricter filtering policies.
By analyzing your bounce data and breaking it down by recipient domain (e.g., gmail.com, outlook.com), you can pinpoint the specific providers that are throttling your mail. Your Email Service Provider (ESP) should provide detailed bounce reports that allow for this level of granularity. If not, consider using external monitoring tools or analyzing SMTP codes to understand the deferral reasons.
Furthermore, it's essential to understand the nature of the email list used for the spike. While you might be sending to engaged users, even engaged lists can suffer if the volume is too high for a short period. Review if the spike involved any new segments, re-engagement of older contacts, or simply a condensed sending schedule for your regular audience. This analysis will help you understand why the soft bounces are occurring and guide your recovery efforts.
Typical causes
Mailbox full, server temporary issues, or greylisting (common for smaller domains). These are often resolvable with retries.
Post-spike issues
Overly aggressive throttling by ISPs due to perceived suspicious volume or a dip in sender reputation. Recipients' mailboxes are not the primary cause after a spike.
Adjusting sending practices and list hygiene
To recover from a volume spike induced soft bounce problem, the immediate action is to scale back your sending rate, especially to the ISPs identified as throttling your mail. Instead of sending all campaigns on a single day, spread your volume across several days. This signals to ISPs that your sending behavior is stabilizing and allows their systems to process your mail without feeling overwhelmed or suspicious. Gradually increasing your volume back to normal, but across a more extended period, is key to rebuilding trust.
Beyond immediate adjustments, maintaining rigorous email list hygiene is paramount. While you may already have a sunset policy (e.g., removing users inactive for 100-140 days), a volume spike can highlight the need for even stricter engagement criteria. Even if the emails aren't turning into hard bounces, persistent soft bounces indicate a quality issue from the ISP's perspective. Consider a more aggressive sunset policy for a temporary period, focusing only on the most engaged subscribers.
Regularly removing inactive subscribers from your lists reduces your overall bounce rate and improves your sender reputation over time. This practice demonstrates to ISPs that you are a responsible sender committed to sending mail only to recipients who genuinely want it. For guidance, refer to best practices for cleaning up soft bounces. Implementing a double opt-in process for new subscribers can also significantly reduce future soft and hard bounces by ensuring all contacts are valid and genuinely interested.
Immediate actions
Identify specific ISPs: Use bounce reports to see which providers are deferring your emails the most.
Reduce sending rate: Temporarily decrease volume to affected ISPs, spreading sends over more days.
Review spike audience: Segment out any less engaged users from the one-day volume spike.
Long-term strategies
Consistent volume: Maintain a steady sending schedule to build and protect sender reputation.
Aggressive sunsetting: Remove inactive subscribers more frequently to maintain a clean list.
Engagement segmentation: Prioritize sending to highly engaged users to improve overall deliverability.
Proactive measures for prevention
To prevent future soft bounce spikes and ensure long-term email deliverability, focus on strengthening your overall email program. A critical aspect is email authentication, specifically SPF, DKIM, and DMARC. These protocols verify that your emails are legitimately sent from your domain, building trust with ISPs. Ensure your DMARC record is properly configured to monitor and protect your sending domain from unauthorized use.
Another preventative measure is adhering to proper email warming practices. If you anticipate a significant increase in sending volume in the future, plan for it by gradually ramping up your sending over days or weeks. This allows ISPs to adjust to your new volume without triggering their protective mechanisms. Avoid large, unannounced volume changes, as these are primary triggers for throttling and reputation issues.
Finally, consistent and proactive list cleaning is non-negotiable. Beyond simply removing unengaged subscribers, consider implementing a regular process for email verification to catch invalid or risky email addresses before you send to them. This will minimize both hard and soft bounces, safeguarding your sender reputation and ensuring your emails reach their intended recipients. Remember, maintaining a high level of sender trust is an ongoing process that requires constant vigilance and adherence to best practices.
Views from the trenches
Best practices
Maintain consistent sending volumes and schedules to avoid triggering ISP throttling mechanisms.
Regularly clean your email lists by removing unengaged subscribers to improve overall email deliverability.
Implement strong email authentication protocols, including SPF, DKIM, and DMARC, to verify your sending legitimacy.
Segment your audience and prioritize sending to highly engaged users to maximize positive signals to ISPs.
Common pitfalls
Ignoring persistent soft bounces, as they can indicate underlying reputation issues that may worsen over time.
Sending to inactive or unengaged users, which can signal low-quality traffic to mailbox providers.
Failing to monitor specific ISP bounce rates, leading to a missed opportunity to target recovery efforts effectively.
Making drastic, sudden changes to email volume without proper warming, which can lead to throttling.
Expert tips
Analyze bounce reports by ISP to identify specific providers causing deferrals, then adjust sending rates for those.
Consider a temporary, more aggressive sunset policy for less engaged segments after a volume spike.
Use email validation services to proactively clean lists and prevent future bounce issues.
Understand that ISP throttling (soft bounces) can be a precursor to worse deliverability problems like blocklisting.
Expert view
Expert from Email Geeks says checking which ISPs generate soft bounces is crucial, as it is unlikely all ISPs started bouncing simultaneously and are continuing to do so.
2020-04-23 - Email Geeks
Expert view
Expert from Email Geeks says backing off your sending rate slightly to those ISPs that are throttling you is a good first step, reducing it until you are not being throttled as much.
2020-04-23 - Email Geeks
Moving forward with better email deliverability
Recovering from a soft bounce spike after a sudden increase in email volume requires a strategic and patient approach. The core issue is often a temporary reputation hit with specific ISPs, which respond to unpredictable sending patterns with throttling. By identifying the affected providers, adjusting your sending rates to them, and committing to consistent, gradual volume, you can begin to rebuild trust.
Long-term success hinges on maintaining impeccable list hygiene, continuously monitoring your sender reputation, and ensuring your email authentication is robust. These proactive measures not only help mitigate the impact of past issues but also lay the groundwork for a stable and high-performing email program. Remember, deliverability is a journey, not a destination, and consistent best practices are the key to keeping your emails in the inbox.
Google and 
Yahoo, closely monitor sending behavior. A drastic, sudden increase in volume, even if the recipients are engaged, can trigger their spam filters or throttling mechanisms. This isn't necessarily a direct blocklisting, but rather a cautionary measure by the ISP to protect their network and users from potential spam or abuse. Effectively managing and reducing these soft bounces requires a multi-faceted approach, focusing on understanding the underlying causes and implementing strategic adjustments to your sending practices and email list hygiene.
