Will a DNS outage impact email deliverability if sends are paused and then resumed?

Key findings

• Minimal impact: For the most part, a DNS outage should not have a significant or lasting impact on email deliverability or sending, especially if sends are paused.
• ESPs and DNS caching: Reputable ESPs typically operate local recursive resolvers that cache DNS records for frequently sent domains, mitigating the immediate effects of an outage.
• Temporary spikes and delays: While some metrics may temporarily spike, or deliveries might be delayed, standard delivery logic is generally equipped to handle these fluctuations without long-term issues.
• Retry mechanisms: Even if an ESP cannot immediately resolve a DNS record, a 'servfail' answer will likely result in the email being queued for retry, rather than a hard bounce.

Key considerations

• DNS record expiration: The only significant risk arises if an ESP's DNS cache for a specific record expires precisely during an outage when primary name servers are unavailable, preventing a fresh lookup.
• Impact on visuals: A DNS outage can sometimes prevent images within emails from loading, as their hosting domains might also be unreachable.
• Traffic surges post-outage: A potential challenge is the surge of backed-up senders attempting to send mail simultaneously after the resolution, which ESPs should manage with proper queuing.
• Understanding DNS issues: For more in-depth troubleshooting of DNS-related delivery failures, review guides like how to troubleshoot intermittent email delivery failures.
• DNS server reputation: Beyond outages, the general reputation of your DNS servers can also play a role in deliverability.

Key opinions

• Recovery time: Marketers frequently question if major mailbox providers like AT&T will instantly recover or if their systems require time to normalize post-outage.
• Image display issues: A common observation during DNS outages is the failure of images to appear in emails, which directly impacts campaign effectiveness.
• Cascading deliverability impacts: There's a concern about whether pausing and then resuming sends immediately after a DNS outage could still lead to unforeseen negative consequences for deliverability.
• Proactive pausing: Many marketers choose to pause sending during any significant service disruption to avoid potential issues and maintain their sending reputation.

Key considerations

• Immediate send resumption: The question remains whether resuming sends immediately after resolution is safe, or if a waiting period is advisable to ensure full system stability.
• Monitoring metrics: It's crucial to closely monitor delivery metrics for any unusual spikes or sustained delays post-outage.
• Branded sending domain setup: Setting up a branded sending domain includes pausing sends, a strategy that can be adapted for outages.
• Impact of repeated pauses: Consider how repeated cycles of massive sends and pauses might affect long-term IP and domain reputation.
• ESP outage management: Understanding how ESPs manage deliverability during service outages is key.

Key opinions

• No inherent deliverability impact: Experts largely agree that a DNS outage, in itself, should not cause a lasting impact on email deliverability or the fundamental mail sending process.
• Role of local resolvers: A crucial factor in mitigating impact is that ESPs typically run local recursive DNS resolvers with caching for frequently accessed domains.
• Temporary metric spikes are normal: It is expected that certain delivery metrics might temporarily spike, but an ESP’s standard delivery logic should handle these without major concern.
• Retry behavior: If DNS resolution fails, mail servers typically return a 'servfail' response, prompting the sending server to retry delivery later.

Key considerations

• DNS cache expiration: A potential problem scenario is if an ESP's cached DNS records expire precisely when the primary name servers are down, preventing resolution.
• Short TTLs and delays: With short Time To Live (TTL) values, such as the five-minute TTLs common with providers like Gmail, ESPs not honoring these or holding onto expired records could experience delayed deliveries.
• Managing traffic surges: The simultaneous release of backed-up email queues by numerous senders after an outage can create temporary congestion, which ESPs must manage effectively.
• Exponential backoff: Implementing exponential backoff for retries is a recommended practice, though not legally mandated, to handle temporary network or DNS issues gracefully.
• Understanding DNS failure rates: For a deeper dive, consider what causes sudden increases in DNS failure and hard bounces in email delivery.
• SPF DNS timeout: Specific DNS issues, like hidden SPF DNS timeouts at Microsoft, can also impact deliverability.

Key findings

• DNS resilience: Internet standards for DNS are built with redundancy and caching to handle minor disruptions without total failure.
• Mail server retries: SMTP protocols explicitly define retry mechanisms for temporary failures (like DNS resolution issues), preventing immediate hard bounces.
• TTL implications: The Time To Live (TTL) value of DNS records dictates how long an ESP can cache information, directly influencing how quickly an outage's impact is felt and resolved.
• SPF and DKIM validation: Authentication protocols (SPF, DKIM) rely on DNS lookups. Outages can temporarily affect their validation, leading to deferrals.

Key considerations

• DNS records for email: MX records are critical for directing email, and DNS outages directly affect their lookup. Proper nameserver changes are key.
• DMARC policy application: DMARC policies are also retrieved via DNS. A failure to retrieve this record might result in mail being treated differently, potentially with DMARC verification failing.
• ISP-specific handling: While standards exist, individual ISPs may have varying thresholds and retry logic for DNS resolution failures, as detailed in their postmaster guidelines.
• Impact on blocklists: DNS-based Blocklists (DNSBLs) are also affected by DNS outages. If a blocklist's DNS is down, it might temporarily fail to provide blocklist data, but this doesn't typically get a sender blacklisted.