Summary
A spike in hard bounces later in email campaigns can arise from a combination of technical delivery factors and email list hygiene issues. Spam filters might react to initial traffic spikes, causing delayed rejections, or the sending pattern itself (e.g., bursts) could appear suspicious. Delivery delays might mean bounces occur later in the send. Importantly, list quality is a key concern: outdated or unengaged addresses are prime candidates for hard bounces, and a less common scenario includes being targeted in a list bombing attack. Proper bounce management, particularly double opt-in, and regular list maintenance are crucial for mitigating these issues. Finally keep in mind 5xx errors indicate hard bounces and these can impact your reputation. A bounce rate spike is a larger problem than a general increase.
Key findings
- Spam Filter Reaction: Spam filters might react to initial traffic spikes, leading to delayed rejections.
- Suspicious Sending Patterns: Sending emails in bursts or specific patterns can appear suspicious to ISPs.
- List Decay: Outdated or unengaged email addresses lead to more hard bounces.
- List Bombing: Being targeted by a list bombing attack can result in significant bounce spikes.
- 5xx Error Codes: 5xx SMTP error codes, such as 550, signal hard bounces.
- Reporting Inaccuracies: Some reporting tools report bounced data in ways that dont make it immediately obvious the timing.
Key considerations
- Analyze Delivery Logs: Review delivery logs to understand spam filter behavior and identify issues.
- List Validation: Implement email validation services to proactively identify and remove invalid addresses.
- Double Opt-In: Employ double opt-in to ensure recipients entered correct information and are willing to receive emails.
- List Cleaning: Regularly clean your email list to remove inactive and invalid addresses.
- Sunsetting Inactives: Implement a policy to sunset and remove inactive subscribers.
- Monitor Bounce Rates: Actively monitor and manage your bounce rates.
- Sender reputation: Manage your sender reputation with regular checks to ensure it isnt impacting deliverability.
What email marketers say
8 marketer opinions
A spike in hard bounces during an email campaign often indicates issues with email list quality. Several factors contribute to this, including outdated email addresses, list decay over time, sending to purchased/scraped lists, and a lack of proper list hygiene practices. Implementing solutions like double opt-in, regular list cleaning, email validation services, and permission-based sending are essential to mitigate these issues and maintain a healthy sender reputation. Normal bounce rates will fluctuate, but should be considered a concern when they spike beyond ~2%.
Key opinions
- Outdated Lists: Email addresses become invalid over time due to job changes, account closures, and other reasons.
- List Decay: Email lists naturally degrade, leading to more hard bounces as subscribers become inactive.
- Poor Acquisition: Purchased or scraped email lists almost always result in high bounce rates.
- No Validation: Lack of email validation and cleaning result in poor deliverability.
Key considerations
- Double Opt-in: Implement double opt-in to ensure only valid and consenting email addresses are added to the list.
- List Cleaning: Regularly clean the email list to remove inactive or invalid email addresses.
- Validation Services: Use email validation services to proactively identify and remove risky email addresses.
- Permission-Based: Only send emails to subscribers who have explicitly given permission to receive them.
- Inactive Sunsetting: Implement a policy to remove inactive subscribers.
Marketer view
Email marketer from Mailchimp shares that regularly cleaning email lists is essential to remove inactive or invalid email addresses. This prevents sending to addresses that will hard bounce, thus preserving sender reputation.
9 Feb 2024 - Mailchimp
Marketer view
Email marketer from StackOverflow explains that bounce rates will always fluctuate. A larger audience will have more drop off. You should be concerned when you suddenly see a big spike, for example > 2%
27 Feb 2023 - StackOverflow
What the experts say
7 expert opinions
A spike in hard bounces later in email campaigns can result from several technical and list-related issues. Spam filters might react to initial traffic spikes, leading to delayed rejections. The deployment method (e.g., bursts of emails) can appear suspicious. Also, hard bounces may simply take time to register. Additionally, list-related problems include sending to old/unengaged addresses, or a list bombing attack where an attacker subscribes the email address to loads of lists. Employing double opt-in as a method of bounce rate management can assist.
Key opinions
- Spam Filter Reaction: Spam filters may react to initial traffic spikes, causing delayed rejections.
- Deployment Suspicion: Burst sending patterns can appear suspicious to ISPs.
- Bounce Delay: Hard bounces can take time to occur and register, affecting later campaign results.
- List Decay: Sending to old or unengaged addresses results in more hard bounces.
- List Bombing: List bombing attacks can cause substantial bounce spikes.
Key considerations
- Delivery Log Analysis: Check underlying delivery logs to confirm spam filter behavior.
- SMTP Bounce Reason: Investigate the reasons for hard bounces.
- Address Timing: Make sure you're sending the correct URN addresses at the right time.
- Address Engagement: Only send email to engaged addresses.
- Opt-in management: Employ double opt-in to ensure recipients entered correct information and are willing to receive mail.
Expert view
Expert from Spam Resource explains that spikes in hard bounces can be attributed to sending to old or unengaged addresses. Over time, email addresses become inactive as people change jobs, switch ISPs, or abandon accounts.
28 Dec 2022 - Spam Resource
Expert view
Expert from Email Geeks suggests the observed hard bounce pattern might be due to spam filters reacting to traffic spikes. He explains that a spam filter may deliver or defer earlier emails and then start rejecting traffic once it detects the spike. Furthermore, he advises checking underlying delivery logs for confirmation.
27 Jan 2024 - Email Geeks
What the documentation says
4 technical articles
Hard bounces are permanent delivery failures caused by issues such as non-existent email addresses, blocked domains, or IP reputation. Spikes in these bounces can significantly impact sender reputation and may lead to alerts or limitations from email service providers like Amazon SES. These failures are typically indicated by 5xx SMTP error codes, such as 550, and understanding these codes is crucial for diagnosing the underlying delivery problems.
Key findings
- Permanent Failures: Hard bounces indicate permanent delivery failures due to issues like invalid addresses or blocked domains.
- Reputation Impact: Spikes in hard bounces negatively impact sender reputation, potentially affecting future deliverability.
- Provider Limits: Email service providers may impose limitations on sending accounts that experience high bounce rates.
- SMTP Error Codes: 5xx SMTP error codes, such as 550, signal hard bounces.
Key considerations
- Bounce Monitoring: Actively monitor bounce rates to identify and address potential deliverability issues.
- Address Validation: Implement measures to validate email addresses and remove invalid ones from mailing lists.
- IP Reputation: Maintain a positive IP reputation to avoid being blocked by receiving servers.
- Error Code Analysis: Analyze SMTP error codes to understand the specific reasons for delivery failures.
Technical article
Documentation from SendGrid Documentation defines a hard bounce as an email that cannot be delivered for permanent reasons, such as a non-existent email address or a blocked domain. Spikes in hard bounces can negatively impact sender reputation.
28 Mar 2025 - SendGrid Documentation
Technical article
Documentation from Microsoft Support details SMTP response codes. A 5xx error, such as 550, indicates a permanent failure, leading to a hard bounce. It's possible your IP may be blocked if the mail is considered harmful.
21 Oct 2021 - Microsoft
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