How do bounces and phishing attacks affect email deliverability and domain reputation?

Key findings

• Temporary vs. permanent bounces: While temporary failures (soft bounces) might not directly harm your reputation, a high volume often signals underlying deliverability problems. Permanent failures (hard bounces, like user unknown) directly damage your sender reputation and lead to blocklistings if not promptly addressed. Regularly removing hard bounces from your lists is paramount.
• Phishing's pervasive impact: Even if phishing attacks aren't originating from your legitimate sending systems, if your domain is being forged or mentioned in phishing content (e.g., in footers or text), it can negatively impact your overall domain reputation. Mailbox providers (MBPs) often use machine learning and manual filters that become wary of domains associated with malicious activity.
• ESP reporting nuances: Different email service providers (ESPs) categorize bounces and blocks differently. It is important to understand their reporting mechanisms, particularly how they differentiate between various bounce types (e.g., temporary, permanent, blocks) and their impact on your standing.
• DMARC protection: Implementing a strong DMARC policy, even at a quarantine level, helps protect your domain from the full reputation impact of forged emails, although it might not completely eliminate the bleed-over effect.

Key considerations

• Identify bounce types: Don't treat all bounces equally. Understand the specific SMTP replies to differentiate between temporary issues and permanent failures like unknown users. This insight helps you prioritize list cleaning efforts and target specific deliverability problems. More on this is available in our guide on hard bounces.
• Maintain rigorous list hygiene: Even with double opt-in, older lists or those not subject to strict verification can contain invalid addresses or spam traps. Regularly cleaning your lists by removing unknown users is critical for improving deliverability and protecting your domain reputation.
• Segment email sending: If you send both transactional and marketing emails, consider using different subdomains or sender identities (e.g., avoid noreply@ for marketing) to prevent issues with one type of mail from contaminating the reputation of the other. This strategy helps isolate potential problems and maintain consistent deliverability. For more on the impact of bounces, refer to this resource from Email Industries.

Key opinions

• Concern over bounce impact: Marketers frequently express concern that high bounce rates directly harm their deliverability, even if their ESP classifies some as temp fails. There's a perceived disconnect between what ESPs report and the observed impact on delivery rates.
• Desire for granular data: Many marketers find it challenging to classify bounces effectively when reports are at an email address level rather than a campaign level, making it difficult to understand specific problems.
• Confusion over ESP suppression: There's often confusion about how ESPs, like SendGrid, manage suppression lists and how different types of bounces (especially blocks) are handled compared to proper bounces.
• Phishing impact on legitimate mail: Marketers are often unaware of how their domain being used in unrelated phishing attacks (e.g., by third-party services) can still impact the deliverability of their legitimate transactional or marketing emails, even with robust internal security.

Key considerations

• Verify ESP bounce classifications: Do not assume your ESP's bounce and block classifications fully align with standard deliverability best practices or the actual impact on your reputation. Always seek deeper insights into the underlying SMTP replies. For details on how bounces affect domain reputation, refer to this article on understanding domain reputation.
• Proactive list cleaning: Implement robust list hygiene practices that go beyond initial double opt-in. Regularly remove all hard bounces (user unknown) and actively manage blocks to mitigate negative reputation impacts. High bounce rates can significantly damage sender reputation and lead to deliverability problems, as explored in articles like The Hidden Costs of Email Bounces.
• Monitor your domain's wider use: Be aware of any external abuse of your domain (e.g., phishing campaigns, even by third parties). While you might not control these entirely, being informed helps you understand potential reputation challenges and enables you to make strategic decisions, such as separating marketing and transactional sending domains. Learn more about how email sending practices impact domain reputation.

Key opinions

• Bounce types matter: Temporary failures may not directly harm reputation but act as a warning signal. Permanent failures, particularly user unknown bounces, are a direct negative deliverability factor. Each MBP might treat different bounce types with varying impacts.
• Phishing's severe domain impact: The prevalent use of a domain in phishing or malware campaigns (even via third-party services) can severely degrade its overall reputation. Machine learning and manual filters become wary of such domains, affecting legitimate mail deliverability, especially at smaller mailboxes.
• DMARC's role in mitigation: While DMARC can help insulate a domain from the full brunt of reputation hits caused by forged messages (especially with a quarantine policy), it may not completely prevent bleed-over reputation issues, particularly if the legitimate domain is mentioned in phishing content.
• Importance of SMTP replies: Relying on generic bounce reports from ESPs isn't sufficient. Senders need to dig into specific SMTP replies to understand the true reason for bounces and the underlying reputation signals (e.g., Yahoo's TS or TSS responses indicate existing poor reputation). This is why understanding reputation drops is critical.

Key considerations

• Action user unknown bounces: Actively remove addresses that consistently return a 550 user unknown response. These signal attempts to mail bad addresses, which negatively impacts reputation. This applies even if you use soft bounce retry policies.
• Separate sending identities: Avoid using noreply@ for marketing newsletters, especially if that address is commonly abused in phishing. Distinct sending domains for different mail streams can help isolate reputation issues.
• Investigate all high bounces: Dig deeper into the specific types of high bounces you're experiencing. A generic bounce count from your ESP doesn't provide enough information to resolve underlying deliverability problems. Getting specific error messages is crucial. For further reading, Mailjet's blog on IP reputation offers insights into related factors.

Key findings

• Bounce rate as reputation indicator: Documentation consistently shows that high bounce rates, particularly hard bounces, are a primary factor in the degradation of sender reputation. They signal poor list quality and can lead to emails being sent to spam or outright blocked.
• Impact of spam complaints: Alongside bounces, documentation frequently cites spam complaints as a direct and severe negative impact on IP and domain reputation. These complaints explicitly tell mail providers that recipients do not want your emails.
• List hygiene importance: Official guides stress the necessity of regular email list cleaning to remove invalid or inactive addresses. This practice is crucial for reducing bounces and avoiding spam traps, which can swiftly lead to blocklisting. This is why tools like our Email Deliverability Tester are important.
• Phishing and domain association: Even if not directly originating from a sender's infrastructure, instances of a domain being exploited for phishing or malware can negatively influence its perceived trustworthiness by recipient systems. This often leads to increased scrutiny and potential filtering of legitimate emails.

Key considerations

• Prioritize bounce rate reduction: Implementing strategies to reduce bounce rates is consistently highlighted as a top priority for maintaining domain reputation and achieving high deliverability. This includes thorough list validation and prompt removal of bounced addresses, as explored in the Benchmark Email guide on reducing bounce rate.
• Implement strong authentication: Beyond managing bounces, documentation consistently recommends robust email authentication protocols such as SPF, DKIM, and DMARC. These measures help verify sender identity and prevent domain spoofing, thereby protecting reputation from phishing attacks.
• Monitor blocklists: Regular monitoring of email blocklists and blacklists is advised to quickly identify if a domain or IP has been listed due to high bounces, spam complaints, or phishing, allowing for prompt remediation. Our blocklist monitoring can help with this.