Will a short DNS TTL (e.g., 300 seconds) make email deliverability worse during an outage?

Yes, a short TTL can worsen email deliverability during an outage because cached records expire more quickly, forcing mail servers to attempt new lookups more frequently. If your DNS is down, these lookups will fail, leading to more temporary failures and delays. For stable records like SPF, DKIM, and DMARC, longer TTLs are generally preferred to leverage caching as a buffer.

How do mailbox providers like Gmail handle DMARC if DNS records aren't accessible?

If DMARC records are inaccessible due to a DNS outage, mailbox providers' behavior can vary. They might issue a temporary failure, allowing the sending server to retry, or, as Google's error message indicates, they may still know the DMARC policy (likely from a cache) but mark the email as unauthenticated if SPF and DKIM fail due to the DNS issue. The ultimate action can depend on the provider's specific implementation and whether they 'fail open' or 'fail closed' on DNS errors, as per RFC 7489.

What's the difference between a DNS server outage and accidental deletion of records?

A DNS server outage means the servers hosting your records are temporarily unreachable, preventing any lookups. Accidental deletion of records, on the other hand, means the records themselves are gone from your DNS configuration, even if the servers are operational. Both scenarios lead to authentication failures, but deletion can persist longer if not detected and corrected quickly. Monitoring tools can help detect both issues swiftly.

Can Suped help me monitor for DNS-related email authentication failures?

Absolutely. Suped is designed to provide comprehensive DMARC monitoring and reporting, which includes identifying SPF and DKIM authentication failures caused by DNS issues, such as temporary errors or missing records. Our AI-powered recommendations help you pinpoint the root cause and take corrective action, ensuring your email deliverability remains high.

What happens to DMARC, SPF, and DKIM during DNS server outages? - DMARC - Email authentication - Knowledge base

DNS, or the Domain Name System, is the internet's phonebook. For email, it's absolutely fundamental, acting as the backbone for critical authentication protocols like SPF, DKIM, and DMARC. When DNS servers experience outages, even brief ones, the implications for your email deliverability and security can be significant.

I often think about the rare, catastrophic scenarios, like what happens if DNS servers go down for a few minutes due to an unexpected reboot. Does DNS caching save the day, or do emails start bouncing with temporary failures? Understanding this interaction is key to building a resilient email infrastructure.

These authentication protocols rely on mail servers being able to look up specific TXT records in your domain's DNS. If those lookups fail, legitimate emails can be treated as suspicious, rejected, or sent to spam folders, eroding your sender reputation and impacting communication. It's crucial to understand how SPF, DKIM, and DMARC work to truly grasp the impact of a DNS outage.

DNS outages and email authentication

When a DNS server experiences an outage, mail servers attempting to perform authentication checks for incoming email may not be able to retrieve the necessary records. The immediate effect largely depends on the Time To Live (TTL) value set for your DNS records and whether the recipient's mail server has a cached copy of your records.

For short outages, if the DNS record was recently queried by the recipient's mail server, the cached entry might still be valid, allowing authentication to proceed without issue. However, if there's no recent query, or if the outage exceeds the TTL, the mail server will try to perform a new lookup. Inability to reach the authoritative DNS server for a domain will typically result in a temporary failure, often an SMTP 4xx error (e.g., a 451 response).

While a temporary failure (tempfail) usually means the sending mail transfer agent (MTA) will retry sending the email later, it can still lead to delays, and in some cases, persistent retries can impact your sending reputation. Furthermore, a mailbox provider might interpret a DNS resolution failure as a missing record, potentially bypassing authentication entirely or applying more stringent filtering.

The specifics of how a mail receiver handles DNS errors when querying for the DMARC policy record are left to their discretion, as outlined in RFC 7489. Some may temporarily reject messages, inviting the sender to retry, while others might deliver the message unauthenticated to minimize disruption, effectively failing open. This variability makes robust DNS critical.

Impact on DMARC, SPF, and DKIM

Protocol	DNS Record Type	Impact of DNS Outage
SPF	TXT (containing authorized IPs)	SPF validation fails. Typically a tempfail, leading to retries or unauthenticated delivery.
DKIM	TXT (containing public key)	DKIM verification fails. Public key cannot be retrieved, resulting in tempfail or unauthenticated status.
DMARC	TXT (containing policy)	Recipient server behavior varies. May default to p=none, or result in temporary rejection if policy isn't cached.

For SPF, if your DNS servers are down, mail receivers can't look up the SPF TXT record for your sending domain. This means they can't verify if the sending IP address is authorized. The result is typically an SPF TempError, inviting a retry. However, repeated temporary errors can lead to longer delivery delays or even rejections.

Similarly, DKIM relies on DNS to retrieve the public key needed to verify the email's digital signature. If the DNS outage prevents the mail server from fetching your DKIM record, the DKIM validation will fail. This is also often treated as a temporary failure, prompting retries. But again, consistent failures can lead to messages being marked as unauthenticated.

For DMARC, the situation is a bit more nuanced. While the DMARC policy itself is a DNS record, some mailbox providers might cache DMARC policies with different logic than the underlying SPF or DKIM authentication records. This could mean a DMARC policy is still 'known' even if underlying authentication checks fail due to DNS issues.

A common response from

Google's (Gmail) servers during DNS failures is: 451-4.7.26 Unauthenticated email from example.com is not accepted due to domain's DMARC policy, but temporary DNS failures prevent authentication. Please contact the administrator of example.com. domain if this was a legitimate mail. To learn about the DMARC initiative, go to Google's DMARC rejection page. This indicates the DMARC policy was found, but SPF and/or DKIM authentication failed due to temporary DNS issues.

Mitigating risks and best practices

Given the critical role of DNS, making it robust is paramount. You should aim for at least two independent DNS servers on independent networks, ideally leveraging an Anycast setup from a large provider. This redundancy helps ensure that if one server or network path fails, others can take over seamlessly. It's also vital to monitor your DNS health consistently, not just your email authentication records.

Another crucial aspect is managing TTL values. While a short TTL of 300 seconds (5 minutes) might seem useful for planned infrastructure changes, it can exacerbate issues during unexpected outages. A longer TTL means cached records remain valid for a longer period, providing a buffer against brief DNS disruptions. For stable records like SPF, DKIM, and DMARC, a longer TTL is generally recommended unless you're anticipating a DNS migration.

Fragile DNS Setup

Single point of failure: Relying on one DNS server or provider leaves you vulnerable to outages.
Short TTL values: Frequent lookups and quicker expiration of cached records increase impact during outages.
Manual monitoring: Waiting for issues to be reported means delayed detection and response.

Robust DNS Setup

Redundant infrastructure: Multiple, geographically dispersed DNS servers with Anycast support.
Optimized TTL values: Longer TTLs for stable records to leverage caching more effectively.
Automated monitoring: Real-time alerts for DNS resolution issues and authentication failures.

Beyond server outages, a more common scenario I've encountered is the accidental deletion or misconfiguration of DNS records. This often leads to the same outcome as an outage, with authentication failing because records can't be found. Proactive monitoring helps here significantly, as you can catch these issues before they severely impact your email program.

This is where a robust DMARC monitoring tool comes into play. Suped provides AI-powered recommendations and real-time alerts for DMARC, SPF, and DKIM issues, including DNS resolution problems. Our platform gives you unified visibility into your authentication status, helping you quickly identify and resolve issues, whether they stem from an outage or an accidental record change. We also offer SPF Flattening and a generous free plan, making it accessible for everyone.

Views from the trenches

Best practices

Implement multiple independent DNS servers on diverse networks to ensure redundancy and uptime.

Use a robust Anycast DNS provider for high availability and distributed global resolution.

Set appropriate TTL values for your email authentication records, balancing caching benefits with update propagation speed.

Common pitfalls

Using a single DNS server, creating a critical single point of failure for email authentication records.

Setting excessively short TTLs without a clear, immediate need, which can amplify the impact of outages.

Accidental deletion or misconfiguration of SPF, DKIM, or DMARC DNS records, which often goes unnoticed.

Expert tips

Automate DNS health checks and integrate them with your alerting systems to detect outages quickly.

Familiarize yourself with mailbox provider-specific handling of DNS failures for authentication records.

Utilize DMARC reporting tools to gain visibility into authentication outcomes and diagnose DNS-related issues.

Expert view

Expert from Email Geeks says that inability to reach an authoritative DNS server for a domain should typically be treated as a temporary failure.

1736812357 - Email Geeks

Marketer view

Marketer from Email Geeks says that many mailbox providers also require A, MX, or AAAA records for the 5321 domain.

1736813709 - Email Geeks

Ensuring email resilience

While DNS server outages for an entire service are rare, the impact on SPF, DKIM, and DMARC, and consequently on email deliverability, can be substantial. The internet's reliance on DNS caching and TTL values offers some resilience, but a proactive approach to DNS infrastructure and monitoring is essential.

By ensuring your DNS is redundant, your TTLs are optimized, and you're actively monitoring your DMARC reports, you can significantly mitigate risks and maintain consistent email deliverability, even in the face of temporary DNS disruptions or accidental record changes. Using a solution like Suped for DMARC monitoring provides the visibility and actionable insights needed to secure your email and protect your domain reputation.