How do ESPs handle DNS caching during an outage?

ESPs typically employ local recursive DNS resolvers that cache records for domains they frequently send to. This caching helps them continue to route email even if public DNS servers are temporarily down, as long as the cached records haven't expired based on their Time To Live (TTL).

Can a DNS outage permanently damage my sender reputation?

No, a DNS outage itself is unlikely to cause permanent damage to your sender reputation if your sending is paused and resumed appropriately. Mailbox providers understand that these are external network issues and generally don't penalize senders for temporary disruptions outside their control. Consistency and good sending practices are key to reputation.

What metrics should I monitor after a DNS outage?

After resuming sends, you should closely monitor your deferral rates, bounce rates, and complaint rates. A temporary increase in deferrals is normal as systems catch up. You should also check for any temporary issues with image loading in your emails if they are hosted on domains also affected by the outage.

How does Time To Live (TTL) affect email during an outage?

TTL determines how long DNS resolvers can cache a record. If an outage lasts longer than the TTL, cached records will expire, forcing resolvers to query unresponsive DNS servers. This can lead to "SERVFAIL" errors and email deferrals until DNS is restored and new records can be fetched.

Will a DNS outage impact email deliverability if sends are paused and then resumed? - Troubleshooting - Email deliverability - Knowledge base

When a DNS (Domain Name System) outage occurs, it can feel like the digital world has come to a screeching halt. For email senders, a primary concern immediately jumps to mind: will this disrupt our email campaigns? More specifically, if we pause our sends during the outage and then resume once the dust settles, what will be the lasting impact on our email deliverability? It's a valid question, as the health of your email program is paramount.

DNS is the internet's phonebook, translating human-readable domain names into IP addresses. For email, it's crucial for looking up MX (Mail Exchanger) records, which tell sending mail servers where to deliver email for a particular domain. If this system goes down, it logically raises concerns about email routing.

I've seen many situations where a DNS outage causes panic, but the reality for email deliverability, especially when sending is paused, is often less severe than anticipated. The key lies in understanding how email systems, particularly those managed by Email Service Providers (ESPs), are designed to handle such transient disruptions.

How DNS outages affect email flow

During a DNS outage, mail servers attempting to deliver email for affected domains won't be able to resolve the necessary MX records. This doesn't usually result in immediate hard bounces. Instead, mail servers will typically queue the emails and attempt to retry delivery later. This retry mechanism is a fundamental part of the SMTP protocol, designed to handle temporary network issues and server unavailability.

The duration of these retries can vary, but generally, they will continue for several hours, or even days, before an email is finally marked as undeliverable with a bounce. This provides a window for the DNS issue to be resolved. You can learn more about what happens to email sent during an outage online. If the DNS records (like MX records) are crucial for your email delivery and are impacted, messages will likely be temporarily deferred with an error indicating a server problem, as outlined in a guide on email sent during an outage.

The critical point is that these are usually temporary failures, not permanent ones. Once DNS resolution is restored, the queued emails will begin to be delivered. The system is designed to be resilient, minimizing the direct loss of emails due to transient DNS problems.

The role of ESPs and DNS caching

Many ESPs operate with robust DNS infrastructure, including local recursive resolvers that cache DNS records. This means that even if public DNS servers experience an outage, your ESP might still have cached copies of the necessary MX records for frequently sent-to domains. This caching can significantly buffer the impact of a short-lived DNS outage.

However, the effectiveness of this caching depends on the Time To Live (TTL) of the DNS records. If a DNS outage lasts longer than the TTL, the cached records will expire, and the ESP's resolvers will then attempt to query the unresponsive authoritative DNS servers. In such cases, the ESP might receive a SERVFAIL answer, prompting the system to defer and retry delivery. It is important to know how to troubleshoot intermittent email delivery failures.

Good ESPs are built to handle these scenarios gracefully. Their internal retry queues and DNS management practices are designed to minimize the impact on their clients' deliverability, even during significant external DNS events. This means that if you paused sending, the ESP's system would be ready to process your emails once DNS resolution is restored.

DNS terms to know

MX record: Specifies mail servers responsible for accepting email messages on behalf of a domain.
Recursive resolver: A DNS server that queries other DNS servers on behalf of a client.
Time To Live (TTL): The amount of time a DNS record is cached by a resolver.
SERVFAIL: A DNS response indicating the name server could not return an answer.

Impact on sender reputation and deliverability

The main concern after a DNS outage, even if sending was paused, isn't typically direct deliverability impact from the outage itself. Instead, it's the potential for a sudden surge of mail if many senders, like yourself, resume their campaigns simultaneously. This surge can temporarily overwhelm receiving mail servers, leading to increased deferrals or temporary blockages. However, receiving systems are generally equipped to handle these spikes, using strategies like greylisting and connection throttling.

Your sender reputation (which dictates your deliverability) is primarily built on consistent sending volume, low complaint rates, low bounce rates, and high engagement. A temporary pause due to an external DNS outage, followed by a controlled resumption, is unlikely to have a lasting negative impact on this reputation. Mailbox providers, such as

Google, are sophisticated enough to differentiate between legitimate sending pauses and malicious activity.

One transient issue you might observe is images not showing in emails immediately after the outage. This happens if image hosts or CDN DNS records were also affected, leading to temporary loading issues. However, this is usually resolved once DNS propagates globally. Understanding email delivery issues can help in such scenarios.

While a DNS outage typically won't permanently damage your sender reputation, it's worth reviewing how pausing email sending affects IP reputation for warmed versus long-term IPs, as context matters. If your mailing strategy includes fluctuating volumes, you may find that it's important to understand how fluctuating hourly email send volume impacts deliverability.

Best practices for resuming sends after an outage

Even if your ESP handles the DNS resilience well, there are steps you can take to ensure a smooth transition back to normal sending operations. One crucial practice is to avoid immediately unleashing a massive backlog of emails right after an outage. While receiving systems can handle surges, a gradual ramp-up is always preferable to maintain optimal deliverability.

Utilize your ESP's delivery logs and monitoring tools. Keep an eye on metrics like bounce rates, deferral rates, and complaint rates immediately after resuming sends. A temporary spike in deferrals is normal as systems catch up, but sustained high rates could indicate a deeper issue. Understanding what a DNSBL is and using a blocklist checker can also be helpful, though an outage itself rarely leads to blacklisting unless compounded by other poor sending practices.

Ensure your DNS records for email authentication (SPF, DKIM, DMARC) are correctly configured and accessible. While a DNS outage means they might have been temporarily unreachable, ensuring they are valid and published once the outage is over is key. You can leverage DMARC monitoring to get reports on your authentication status, which will quickly highlight any lingering issues related to DNS.

In summary, a DNS outage, especially when combined with paused sending, is less likely to cause a lasting deliverability crisis than many fear. The internet's email infrastructure is built with resilience in mind, and most ESPs have mechanisms to absorb these shocks. The focus should be on controlled resumption and diligent monitoring rather than anticipating a severe, long-term impact on your sender reputation or inbox placement.

Views from the trenches

Best practices

Always leverage your ESP's internal DNS caching capabilities to mitigate short-term outages and ensure seamless retries.

Implement a gradual warm-up approach when resuming sending after a significant pause, even if due to a DNS outage, to avoid sudden volume spikes.

Regularly monitor your email deliverability metrics, including deferrals and bounces, to quickly identify and address any post-outage anomalies.

Maintain valid and accessible DNS records for email authentication protocols, such as SPF, DKIM, and DMARC, to ensure consistent mail flow.

Common pitfalls

Attempting to immediately send a large backlog of emails post-outage, which can overwhelm receiving mail servers and trigger throttling.

Failing to monitor deliverability dashboards and DMARC reports for signs of persistent issues, assuming all problems are resolved automatically.

Not accounting for DNS TTL values, which can lead to delayed propagation of restored DNS records and extended deferrals.

Ignoring the temporary impact on non-essential elements like image loading, which can affect user experience even if emails are delivered.

Expert tips

DNS outages typically lead to temporary deferrals rather than hard bounces, allowing mail to queue and retry once services are restored.

Modern email systems, including those at major mailbox providers, have built-in retry mechanisms and DNS caching to handle transient network issues.

While a DNS outage may cause temporary spikes in certain delivery metrics, it rarely results in a lasting negative impact on sender reputation if managed correctly.

Consider implementing exponential backoff strategies for your sending infrastructure to gracefully handle temporary delivery failures.

Expert view

Expert from Email Geeks says a DNS outage shouldn't have any lasting impact on deliverability or mail sending.

2021-07-22 - Email Geeks

Expert view

Expert from Email Geeks says your ESP should have a recursive resolver locally that caches DNS records for regular sending destinations.

2021-07-22 - Email Geeks

Summary of DNS outages and deliverability

The good news is that pausing email sends during a DNS outage and then resuming them is a sound strategy that typically minimizes any lasting negative impact on deliverability. The email ecosystem is built with redundancies and retry mechanisms to handle temporary network issues, including DNS problems.

While you might see temporary spikes in deferrals or minor issues like unrendered images, these are usually short-lived. The core of your sender reputation, which is based on long-term sending habits and recipient engagement, is unlikely to be significantly affected. Focus on a controlled resumption of sending and continuous monitoring to ensure your email program remains healthy.