What could cause a sudden increase in DNS failure and hard bounces in email delivery?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Jul 2025
Updated 19 Aug 2025
6 min read
Experiencing a sudden surge in email DNS failures and hard bounces can be a deeply frustrating and concerning issue for any sender. It indicates a significant problem impacting your ability to reach recipients, potentially crippling marketing campaigns or critical transactional communications. When these numbers jump dramatically over a short period, it's a clear signal that something fundamental has shifted, even if your sending volume and practices haven't visibly changed.
The increase in hard bounces often accompanies DNS failures because if the recipient's mail server cannot be located or verified, the email simply has nowhere to go. This isn't just about temporary delays, like a full inbox. These are often permanent failures that directly impact your domain reputation and could lead to more widespread deliverability issues, including being flagged as spam.
Identifying the root cause requires a systematic approach, looking at both your sending infrastructure and potential external factors. While it might seem like a recipient-side problem, many issues that manifest as DNS failures or hard bounces can originate closer to home.
Decoding DNS failures in email delivery
A DNS failure in the context of email delivery typically means that the sending mail server could not successfully resolve the domain name of the recipient's mail server. This critical step involves querying the Domain Name System (DNS) to find the Mail Exchange (MX) records for the recipient domain. MX records tell sending servers where to deliver email for that domain.
If these records are unavailable, incorrect, or the DNS server itself is experiencing issues, the email transmission cannot proceed. Common DNS-related problems include misconfigured sending domains, temporary outages of the recipient's DNS servers, or issues with DNSSEC or IPv6. A sudden spike in these failures often points to a systemic problem rather than isolated instances.
No MX record: The domain might not have an MX record, or it's incorrectly configured. This means there's no instruction on where to send the email.
DNS timeout: The sending server tried to query the DNS but received no response within a set time, often due to network congestion or an unresponsive DNS server.
NXDOMAIN: The domain name simply does not exist. This is common when email lists contain outdated or misspelled addresses.
The nature of hard bounces
Hard bounces are permanent delivery failures, meaning the email cannot be delivered to the recipient for a lasting reason. Unlike soft bounces, which are temporary issues, a hard bounce implies that further attempts to send to that address will also fail. High hard bounce rates are detrimental to your sender reputation and can lead to your IP or domain being placed on a blacklist.
When a hard bounce occurs due to a DNS failure, it means the recipient's domain could not be found or was unreachable, making the email address effectively non-existent from the sending server's perspective. The challenge with diagnosing a spike in hard bounces is that the term "hard bounce" itself is often a generic classification by an Email Service Provider (ESP), masking the specific reason behind the rejection. To truly understand the problem, you need to dig deeper into the actual bounce codes and messages returned by the receiving mail servers.
A key distinction is whether the hard bounce is because the mailbox simply doesn't exist (a typical permanent error) or if it's a technical issue like a DNS failure. Both result in non-delivery, but the troubleshooting steps differ. If the primary cause reported is DNS failure, then your focus should be on resolving the underlying DNS issues, which in turn will reduce your hard bounce rate.
Common culprits and troubleshooting your domain
When you see a sudden increase in DNS failures and hard bounces, it's natural to suspect issues with your own email setup. While this is often the case, it's crucial to systematically check all potential culprits.
One of the first places to look is your email authentication records: SPF, DKIM, and DMARC. Even if these were previously configured correctly, recent changes to your DNS provider, domain registrar, or email sending platform could inadvertently break them. For example, if a new DNS record for your sending domain was recently added or modified, it might conflict with existing authentication records, causing validation failures.
DNS and authentication issues
Incorrect SPF record: An invalid SPF record can lead to your emails being rejected. For Microsoft domains, an SPF DNS timeout can sometimes occur.
Missing or invalid DKIM: If your DKIM signature is missing or improperly configured, recipients can't verify the email's authenticity.
DMARC policy issues: A DMARC policy set to reject or quarantine with alignment failures will lead to bounces. Ensure your DMARC, SPF, and DKIM records are aligned and valid.
Incorrect MX records: While DNS failure often refers to the recipient's MX record not being found, a misconfigured MX record on your sending domain (if you're self-hosting) can also cause issues. For example, intermittent delivery failures can stem from these DNS issues.
It is also worth investigating if any recent IT changes, particularly those related to IPv6 readiness or DNSSEC implementation (for example, to meet new sender requirements for providers like Yahoo and Gmail), have inadvertently disrupted your DNS setup. Even minor tweaks can have significant ripple effects on email deliverability.
Mitigating external and recipient-side issues
While your domain's DNS configuration is a primary suspect, sudden increases in DNS failures and hard bounces can also stem from factors beyond your direct control, or from subtle shifts in your sending patterns that trigger recipient server defenses.
Recipient server issues, such as a temporary outage, overload, or network congestion at the receiving end, can certainly lead to DNS timeouts and subsequent hard bounces. While these are usually temporary, a widespread or prolonged issue at a major Mailbox Provider (MBP) could explain a sudden, high volume of failures. Similarly, an invalid or non-existent email address for the recipient is a classic cause of hard bounces. If your email list hygiene has slipped, or you've recently added a large segment of unverified addresses, this could be the culprit.
Proactive measures
Validate email lists: Regularly clean your email lists to remove invalid or inactive addresses.
Monitor sending volume: Avoid sudden, large increases in sending volume, as this can trigger spam filters and lead to bounces and even blocklisting. Your bounce rates can be impacted.
Check ISP feedback loops: Monitor feedback loop reports from major ISPs to identify complaint trends early.
Views from the trenches
Best practices
Actively monitor your bounce logs for specific error codes to understand the precise reasons for rejections.
Regularly verify your email authentication records (SPF, DKIM, DMARC) for correctness, especially after any DNS changes.
Maintain meticulous list hygiene, removing non-existent or inactive email addresses to prevent hard bounces.
Segment your audience and warm up new IP addresses or domains gradually to build a positive sending reputation.
Common pitfalls
Not differentiating between hard and soft bounces, leading to continued sending to invalid addresses.
Ignoring specific bounce error messages provided by your ESP, which contain valuable diagnostic information.
Failing to review recent DNS changes or network updates that might affect your email infrastructure.
Sending to old, unengaged, or purchased email lists without prior validation, causing significant bounce spikes.
Expert tips
Use an email testing tool to get a comprehensive report on your DNS and authentication setup.
If you suspect recipient-side issues, try sending a small test batch to known good addresses on problematic domains.
Consult with your ESP's deliverability team for deeper insights into bounce classifications and logs.
Implement DMARC with reporting to gain visibility into authentication failures and potential spoofing attempts.
Marketer view
Marketer from Email Geeks says that if you have little information, the increase in hard bounces could mean that you acquired a lot of bad email addresses.
February 14, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that you should look at what is common in those bounces, such as recipient MX records or the source of the emails, to identify any technical reasons on your end.
February 14, 2024 - Email Geeks
Troubleshooting steps and prevention
A sudden increase in DNS failures and hard bounces is a critical deliverability signal that demands immediate attention. By systematically investigating your own DNS and authentication records, analyzing detailed bounce reports from your ESP, and considering external factors, you can pinpoint the cause. Resolving these issues quickly will not only restore your email deliverability but also protect your valuable sender reputation, ensuring your messages consistently reach their intended inboxes.
Incorrect SPF record: An invalid SPF record can lead to your emails being rejected. For Microsoft domains, an SPF DNS timeout can sometimes occur.
Missing or invalid DKIM: If your DKIM signature is missing or improperly configured, recipients can't verify the email's authenticity.
DMARC policy issues: A DMARC policy set to reject or quarantine with alignment failures will lead to bounces. Ensure your DMARC, SPF, and DKIM records are aligned and valid.
Yahoo and 
Gmail), have inadvertently disrupted your DNS setup. Even minor tweaks can have significant ripple effects on email deliverability.
