How do bounces and phishing attacks affect email deliverability and domain reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 Jun 2025
Updated 15 Aug 2025
7 min read
Email deliverability is about more than just getting your emails sent, it is about ensuring they reach the recipient's inbox rather than being rejected or sent to spam folders. Two critical factors that can severely impact this are email bounces and phishing attacks. These issues directly influence your domain reputation, which email providers (ISPs) use to decide whether to accept your messages. A tarnished reputation can lead to significant problems, from low delivery rates to outright blocking of your emails.
Understanding how bounces are classified and the subtle ways phishing can damage your brand, even if not directly originating from your systems, is key to maintaining a healthy sending posture. High bounce rates, especially hard bounces, signal poor list hygiene to ISPs. Meanwhile, your domain's association with phishing can degrade its perceived trustworthiness, making it harder for your legitimate emails to land in the inbox.
The impact of email bounces on deliverability
Email bounces are essentially non-delivery notifications, indicating that an email could not be delivered to its intended recipient. They come in two primary forms: soft bounces and hard bounces. While both represent a failed delivery, their underlying causes and impact on your domain reputation differ significantly.
Hard bounces are permanent delivery failures, often due to an invalid or non-existent email address, or an outdated domain. Repeatedly sending emails to these addresses is a red flag for ISPs and will severely damage your sender reputation, as noted by email deliverability experts. ISPs interpret high hard bounce rates as a sign that you are sending to purchased or old, unengaged lists, which is characteristic of spamming. Many email service providers automatically suppress hard-bounced addresses to protect your sending reputation, but you should always confirm this behavior.
Soft bounces are temporary delivery failures. They can occur if the recipient's mailbox is full, the server is temporarily unavailable, or the email size exceeds limits. While a single soft bounce isn't as damaging as a hard bounce, a consistently high rate of soft bounces for certain domains or campaigns suggests underlying issues. For example, some anti-spam systems might temporarily bounce emails if their indicators suggest rejection, even if it's a false positive. While many ESPs will retry sending soft bounces, if you continue to see a high volume of these for a particular segment, it may indicate a problem with your sending practices or a deteriorating reputation at that specific mailbox provider.
Hard bounces
Cause: Permanent failure, such as an invalid or non-existent email address or domain. Examples include addresses like user@nonexistentdomain.com.
Impact on reputation: Significant negative impact on email deliverability and sender reputation. High rates indicate poor list quality and can lead to emails being blocked or flagged as spam.
Management: Immediately remove these addresses from your lists to prevent further damage. Regularly clean your email list.
Soft bounces
Cause: Temporary failure, such as a full inbox, server downtime, or large email size. It might also be a deliberate bounce response from an anti-spam filter.
Impact on reputation: Less immediate, but consistent soft bounces can signal poor engagement or content issues. While a few soft bounces might not directly affect reputation, they can be a warning sign.
Management: Monitor trends. Investigate patterns if a specific domain or campaign experiences high soft bounce rates. Your ESP might retry these automatically. Ensure you don't keep sending to addresses that consistently soft bounce over a period of time.
The unseen dangers of phishing attacks
Phishing attacks pose a severe threat to your domain's reputation, even if your own systems aren't directly compromised. These attacks often involve bad actors impersonating legitimate brands and domains to trick recipients. When your brand's domain (or a very similar cousin domain) is used in phishing attempts, it negatively affects how ISPs view all email originating from that brand. This phenomenon is known as brand impersonation.
Internet Service Providers and email clients use advanced filters, including machine learning algorithms, to detect and block phishing attempts. If your domain is frequently associated with phishing, even indirectly, these filters will become wary of all your mail. This can lead to legitimate emails being quarantined, sent to spam, or outright rejected, regardless of your good sending practices. For example, if a well-known service like WeTransfer has its domain frequently forged in phishing campaigns, mail servers will develop a heightened scrutiny for any emails mentioning that brand, regardless of the sender.
The silent impact of phishing on your domain
When malicious actors impersonate your brand or domain in phishing campaigns, it erodes trust with ISPs and recipients. Even if these phishing emails aren't sent from your actual infrastructure, the association can lead to your legitimate emails being filtered. This is particularly damaging for transactional emails or critical business communications, as their delivery can be inadvertently affected. This can also result in your IP address being on a blacklist (or blocklist). Your domain might also be put on a domain blacklist.
Implementing robust authentication protocols like DMARC, SPF, and DKIM is crucial to protect your brand from spoofing and phishing. DMARC policies, particularly with enforcement (quarantine or reject), help mailbox providers identify and handle unauthorized emails impersonating your domain. While DMARC can't prevent all phishing, it significantly reduces the impact of such attacks on your domain's reputation. Monitoring your DMARC reports is vital for understanding how your domain is being used across the internet.
Even with strong email authentication, the constant vigilance of ISPs means your domain can still be affected. They consider not just direct sending behavior but also the broader landscape of how your brand appears in email traffic. Therefore, managing your domain's online presence and being aware of any potential misuse is an ongoing task.
Blocklists (blacklists) and spam traps
High bounce rates, particularly hard bounces to invalid addresses, are a primary reason domains end up on email blocklists (or blacklists). Blocklists are databases used by ISPs to identify and block mail from senders associated with spam or malicious activity. When your domain (or IP) lands on a blocklist, your emails will likely be rejected or routed directly to spam folders by many major mailbox providers.
Spam traps are a severe form of blocklist trigger. These are inactive or fake email addresses used by ISPs and blocklist operators to identify spammers. Hitting a spam trap indicates that your list acquisition practices are poor or that your lists are outdated. Even if you employ double opt-in, older lists not regularly cleaned can contain recycled spam traps. Sending to just a few spam traps can instantly tank your domain reputation and lead to listing on critical blocklists.
The relationship between bounces, phishing, and blocklists is cyclical. Poor list hygiene leads to bounces, which then harm your reputation and increase your chances of hitting spam traps or being listed on a blocklist. Similarly, consistent phishing activity associated with your brand can also prompt ISPs to treat your legitimate emails with suspicion, leading to rejections that might appear as bounces or simply contribute to a lower delivery rate.
Issue
How it affects deliverability and reputation
Mitigation strategy
Hard bounces
Signals poor list hygiene; ISPs may block or throttle your sending. Leads to a damaged sender reputation.
Regularly clean your email lists and promptly remove invalid addresses.
Soft bounces
Indicate temporary issues, but persistent soft bounces can suggest content problems or a declining sender score with specific ISPs.
Monitor SMTP responses to understand the root cause. Implement retry policies for temporary failures.
Phishing attacks
Even if not from your infrastructure, brand impersonation erodes trust, causing legitimate emails to be filtered or blocked due to association. Damages long-term domain reputation.
Implement DMARC with a strict policy (quarantine or reject) to prevent unauthorized use of your domain. Monitor DMARC reports.
Spam traps
Immediate and severe damage to reputation, leading to blocklisting and significant deliverability issues.
Avoid purchased lists. Use double opt-in. Regularly cleanse your lists of unengaged or old contacts.
Strategies to protect your domain reputation
Protecting your domain reputation from bounces and phishing requires a multi-faceted approach centered on proactive management and adherence to email best practices. Implementing these strategies is crucial for ensuring your emails consistently reach their intended recipients.
Key strategies for a strong sender reputation
List hygiene: Regularly clean your email lists to remove inactive, invalid, or non-existent addresses. A high bounce rate signals poor email hygiene to ISPs. Implement a double opt-in process for new subscribers.
Email authentication: Properly configure SPF, DKIM, and DMARC records. DMARC, especially with enforcement policies (p=quarantine or p=reject), helps prevent unauthorized use of your domain in phishing attacks and instructs recipient servers on how to handle unauthenticated emails from your domain.
Content quality: Avoid spammy keywords, excessive images, or broken links. Personalize your content and provide clear value to your recipients. Regularly test your email content against spam filters.
Engagement monitoring: Track metrics like open rates, click-through rates, and unsubscribe rates. Low engagement can signal to ISPs that your emails are not desired, potentially leading to filtering or even rejections. Inconsistent sending volume can also negatively impact your sender score.
Sender address: Avoid using generic addresses like noreply@yourdomain.com for marketing emails. Use clear, branded sender names and email addresses to foster trust and distinguish your legitimate communications.
Regularly monitor your email deliverability and domain reputation using tools that provide insights into bounce rates, spam complaints, and blocklist status. This proactive approach allows you to identify issues early and take corrective action before they severely impact your email program.
Remember, your email program's success is intricately tied to your domain's reputation. A healthy reputation means your messages reach the inbox, fostering better engagement and protecting your brand. Neglecting bounces or ignoring the threat of phishing can lead to significant deliverability challenges and long-term damage to your domain's trustworthiness.
The path to better email deliverability
Maintaining a strong email deliverability is an ongoing process that demands attention to detail and continuous optimization. By understanding the nuances of bounces and the pervasive threat of phishing, you can implement effective strategies to safeguard your domain reputation and ensure your emails consistently reach their intended audience. Proactive monitoring and adherence to email best practices are not just good ideas, they are essential for your email program's success.
Views from the trenches
Best practices
Actively monitor your bounce reports, categorizing bounces to understand their root causes and taking appropriate action.
Implement a strict DMARC policy (p=quarantine or p=reject) to protect your domain from being used in phishing attempts and prevent unauthorized email sending.
Regularly clean your email lists, removing unengaged subscribers and addresses that result in hard bounces.
Use distinct 'from' addresses for different email streams, such as marketing vs. transactional emails, to better manage reputation.
Common pitfalls
Assuming all bounces are treated equally by ISPs, when permanent bounces (hard bounces) are far more damaging than temporary ones.
Not recognizing that your domain's reputation can be negatively impacted by phishing attempts that spoof your brand, even if they aren't sent from your systems.
Failing to remove addresses that consistently hard bounce, leading to a degraded sender score and potential blocklisting.
Relying solely on your ESP's default bounce handling without understanding their specific classification and suppression mechanisms.
Expert tips
Dig into the SMTP reply codes for bounces; they provide crucial context on why a message failed to deliver and can guide your remediation efforts.
Understand that some ISPs deliberately provide vague bounce responses to spammers, making it harder for them to diagnose and bypass filters.
Be aware that hitting spam traps, often disguised as invalid addresses, can severely impact your domain reputation even if no explicit bounce notification is received.
A comprehensive DMARC strategy, coupled with vigilance against brand impersonation, is key to mitigating the 'bleed over' of negative reputation from phishing.
Expert view
Expert from Email Geeks says if temporary failures are listed in bounce reports, they don't directly harm reputation, but a high volume of them signals that the overall reputation is already poor.
October 1, 2020 - Email Geeks
Marketer view
Marketer from Email Geeks says some anti-spam systems may bounce emails if their indicators suggest rejection, which could include false positives, and some systems count repeated sending attempts even after a bounce.
