How to manage hard bounced email addresses for future sends?

Key findings

1. Immediate suppression: Most email service providers (ESPs) and experts recommend immediately suppressing or removing hard bounced email addresses from marketing lists to protect sender reputation.
2. Transactional email nuance: For transactional emails (e.g., receipts, password resets), some argue for re-attempting delivery or attempting to contact the user through alternative channels due to potential legal or service obligations. This is often a judgment call depending on the criticality of the message.
3. Spam trap risk: Sending to invalid email addresses, even after a hard bounce, significantly increases the risk of hitting spam traps, which can lead to severe blocklisting of your IP or domain.
4. Reputation impact: High hard bounce rates signal poor list hygiene and can negatively impact your sender reputation, making it harder to reach the inbox for legitimate emails.
5. Definition consistency: The definition of a hard bounce and the messages returned can vary between mail providers, complicating automated management.

Key considerations

1. List hygiene: Proactive list cleaning and email validation are essential to prevent hard bounces in the first place, rather than reacting to them. Regularly removing bad email addresses is a cornerstone of good deliverability.
2. Automated suppression: Most ESPs automatically suppress hard bounces, but understanding your platform's specific policy is important to ensure compliance with best practices.
3. User communication: For critical communications where an email hard bounces, consider alternative contact methods (e.g., in-app notifications, phone calls) to inform the user about the delivery issue and prompt them to update their email address. Read more about handling bounces from MessageFlow.
4. Defining hard bounce: Clearly defining what constitutes a hard bounce for your organization (e.g., immediate failure, multiple failures over time) can help streamline bounce management policies.

Key opinions

1. Indefinite suppression: Many marketers advocate for indefinite suppression of hard bounced email addresses from mailing lists. This means once an email hard bounces, it's removed and not sent to again, unless the recipient explicitly re-subscribes or requests to be added back.
2. Reputation priority: The primary driver for immediate suppression is to protect sender reputation. Repeatedly attempting to send to invalid addresses can lead to increased bounce rates, negatively impacting inbox placement for all other emails.
3. Transactional exceptions: For transactional emails (e.g., order confirmations, account notifications), marketers may consider a more lenient approach or alternative contact methods if the user is actively engaging with the business through other channels. This balances deliverability with customer service needs.
4. The 're-deliverable' debate: While some theories suggest a hard bounced address might become deliverable again after several months, most marketers find the risk to sender reputation too high to test this theory. Mailchimp's documentation provides further insight into hard vs. soft bounces.

Key considerations

1. User intent: Consider the user's intent when the email address was provided. If it was for a critical service or interaction, the approach to a hard bounce might differ compared to a newsletter subscription.
2. Alternative communication: If you cannot reach a customer via email, explore other communication channels (e.g., in-app messages, SMS) to notify them about the issue and encourage an email update. For more on managing inactive subscribers, see our guide.
3. System automation: Ensure your email sending platform automatically handles hard bounces by suppressing them, thus preventing further sends and protecting your sending reputation.

Key opinions

1. Contextual definition: Experts emphasize that how you define 'hard bounce' and the context of the bounce event significantly influence the appropriate response. What constitutes a hard bounce might be stricter for some senders than others.
2. Triple bounce rule: A common expert recommendation for marginal cases is to suppress an address if it's rejected at least three times over a period like fourteen days. More aggressive policies, such as 'one bounce and gone', are also considered acceptable for strict list hygiene.
3. Spam trap effectiveness: While recycled spam traps (old addresses repurposed as traps) exist, widespread blocking due to hitting them is often a misinterpretation. ISPs typically use such hits as indicators of poor address acquisition, not necessarily an immediate block trigger for a single instance. Learn more about different types of spam traps.
4. Non-standard bounce errors: Bounce error messages are not standardized, and some providers deliberately return misleading messages (e.g., throttling disguised as connection errors). This makes automated parsing and response more challenging.
5. Transactional versus marketing: Transactional emails often have more flexible compliance requirements. If a hard bounce occurs for a transactional email, attempting to contact the user via other channels (e.g., website, ticketing system) is recommended before re-attempting email.

Key considerations

1. Automated suppression for marketing: For marketing lists, suppressing hard bounces should be an immediate, automated process to safeguard domain reputation. Repeatedly hitting invalid addresses is a clear signal of poor list quality.
2. Judgment calls for transactional: Deciding whether to retry a transactional email after a hard bounce requires a judgment call based on the criticality of the email and other user interaction patterns. This differs from soft bounce retry policies.
3. Email validation tools: While not a panacea, email validation tools can be useful for periodically cleaning lists of suspected undeliverable addresses, especially if you consider re-engaging old contacts. Mailgun discusses what to do about them.

Key findings

1. Permanent failure indication: Documentation defines hard bounces as permanent delivery failures, often due to invalid email addresses, non-existent domains, or blocked recipients.
2. Automatic suppression: Most ESPs automatically suppress (or clean) hard bounced email addresses from your audience to prevent future sends and protect your reputation.
3. Reputation protection: The primary reason for suppressing hard bounces is to safeguard the sender's reputation with Internet Service Providers (ISPs) and prevent their emails from being flagged as spam.
4. No re-sends: General advice from documentation is not to resend emails to addresses that have hard bounced, as they are unlikely to become deliverable and pose a risk.

Key considerations

1. Monitoring bounce rates: Documentation often advises monitoring bounce rates as a key deliverability metric. High hard bounce rates indicate issues with list acquisition or maintenance that need to be addressed.
2. List cleaning practices: Regularly cleaning your email list to remove invalid or unengaged subscribers is a recommended best practice to minimize hard bounces.
3. Distinguishing bounce types: Understanding the difference between hard and soft bounces is critical, as they require different management strategies. Soft bounces might warrant retries, while hard bounces do not. Learn more about the differences from Klaviyo.