Summary
Email Service Providers (ESPs) manage deliverability during outages at major providers like Yahoo or AOL by treating these events as temporary failures, classifying them as soft bounces. They employ sophisticated retry mechanisms, which involve queuing emails and systematically re-attempting delivery for a set duration, often up to 72 hours. This process frequently includes strategies such as exponential backoff, where the time between retries increases, and a reduction in concurrent outbound connections to avoid overwhelming the recovering infrastructure. This adaptive and persistent approach ensures that legitimate emails are eventually delivered once the receiving servers return to full operation, minimizing message loss during transient network or server disruptions.
Key findings
- Outage Characteristics: Outages affecting major providers like Yahoo and AOL can stem from diverse technical issues, including DNS failures, internal SMTP transaction timeouts, and TLS handshake errors, indicating complex underlying problems rather than simple server downtime.
- Regional Impact: The effects of an outage are often not uniform, with some geographical regions or specific domain extensions (e.g., yahoo.com versus yahoo.de, yahoo.fr, or yahoo.co.in) recovering at different rates, necessitating a nuanced approach to re-sending.
- Dynamic Error Codes: During an outage, the error messages returned by affected servers can evolve from temporary service unavailability codes (e.g., 421 Service not available) to other transient or even hard bounce indicators as the situation progresses towards resolution.
- Progressive Recovery: Delivery rates for impacted domains typically show a gradual, incremental improvement over time, with ESPs observing steadily rising delivery percentages as the affected services are progressively restored.
Key considerations
- Soft Bounce Classification: ESPs must classify delivery failures due to ISP outages as 'soft bounces' rather than permanent failures, enabling the system to automatically re-attempt delivery for these temporary issues over a set period, typically up to 72 hours.
- Robust Retry Logic: Implement comprehensive retry mechanisms that queue emails for affected domains and systematically re-attempt delivery for an extended period, ensuring messages are not prematurely discarded due to transient network or server issues.
- Exponential Backoff: Utilize progressive or exponential backoff strategies, increasing the delay between retries to avoid overwhelming recovering servers and allowing them sufficient time to stabilize, a practice highly beneficial for re-establishing deliverability.
- Connection Management: During severe outages, consider proactively reducing outbound concurrent connections to affected domains, even to as low as one connection per queue, to alleviate pressure on the recovering infrastructure and facilitate a smoother recovery.
- Continuous Monitoring: Actively monitor delivery rates, specific SMTP error codes, and regional performance for affected domains to adapt sending strategies in real time, ensuring optimal deliverability as services gradually return to normal.
What email marketers say
19 marketer opinions
During service outages at major providers like Yahoo or AOL, Email Service Providers effectively manage deliverability by activating comprehensive deferral and retry systems. They queue emails, applying sophisticated logic that includes progressively extending retry intervals and, at times, reducing outbound connection rates to prevent overwhelming the recipient server. This adaptive approach ensures that messages, even those initially encountering temporary errors, are persistently re-attempted and ultimately delivered as the affected services regain full functionality, demonstrating ESPs' resilience in maintaining email flow during transient disruptions.
Key opinions
- Specific Error Types Observed: Beyond general server issues, outages manifest with specific error codes like 'StatusDnsQueryFailed', '421 Service not available', and '403 4.7.0 TLS handshake failed', indicating diverse technical challenges.
- Non-Uniform Regional Recovery: Recovery is often staggered, with some domains, for example, yahoo.com, stabilizing quickly while others, particularly in EU and APAC regions such as yahoo.de, yahoo.fr, and yahoo.co.in, experience prolonged or differential issues.
- Gradual Delivery Improvement: Delivery rates for impacted domains show a progressive increase, moving from significant failures to high success rates, for example, 70% to over 96%, indicating a phased return to service.
- Evolving Server Responses: During an outage, the nature of error messages can change, such as transitioning from temporary service unavailability to more definitive error types as the recipient server's status evolves, which ESPs must interpret correctly.
Key considerations
- Proactive Connection Limiting: During severe outages, actively limit outbound concurrent connections to affected domains, potentially to one per queue, to ease pressure on the recovering infrastructure and aid faster restoration.
- Dynamic Error Interpretation: Continuously monitor and interpret changing SMTP error messages, as the evolution of these codes, for example, from 421s to hard bounces, or the absence of TLS errors, provides vital clues about the recovery status and guides retry adjustments.
- Strategic Traffic Management: Implement a deliberate strategy to back off or temporarily pause email traffic to severely affected domains, going beyond automated retries to actively manage sending volume during critical recovery phases.
- Gradual Return to Normal: Once recovery is confirmed and delivery stabilizes at high rates, ESPs should incrementally restore normal sending parameters, acknowledging the role of sender cooperation in the recovery process.
Marketer view
Email marketer from Email Geeks explains that Yahoo & AOL DNS were down and that the service was back in a little less than 40 minutes.
6 May 2022 - Email Geeks
Marketer view
Email marketer from Email Geeks explains that the Yahoo & AOL issue was not just DNS, as half of SMTP transactions were failing due to internal timeouts and outward-facing servers not reaching the backend. They later noted .com was fully functional, but EU zones were still 50/50 without TLS errors.
30 Apr 2025 - Email Geeks
What the experts say
2 expert opinions
When major email providers like Yahoo or AOL experience service disruptions, Email Service Providers are tasked with carefully handling the influx of temporary delivery failures. Their strategy involves classifying these as soft bounces, queuing the affected messages, and consistently retrying delivery over an extended period, typically up to 72 hours. This persistent approach ensures emails are delivered once the receiving systems stabilize, preventing premature hard bounces or unwarranted unsubscribes.
Key opinions
- Temporary Bounce Handling: Email Service Providers should anticipate a high volume of temporary bounces during ISP outages and must avoid immediately categorizing these as permanent failures.
- Extended Retry Duration: It is standard practice for ESPs to hold onto messages and continuously retry delivery for an extended period, typically around 72 hours, during service disruptions.
- Soft Bounce Classification: Messages impacted by temporary ISP unavailability should be classified as soft bounces and queued, ensuring persistent delivery attempts based on temporary error codes, such as 4xx.
- Prevent Premature Actions: ESPs must avoid prematurely treating temporary failures as permanent bounces or unsubscribing users, instead patiently retrying delivery until ISP systems recover.
Key considerations
- Accurate Bounce Classification: It is critical for ESPs to accurately classify delivery issues during ISP outages as temporary, or soft bounces, preventing their systems from prematurely marking these as permanent failures.
- Sustained Delivery Retries: ESPs must implement robust mechanisms to queue messages and sustain delivery retries for an extended duration, typically up to 72 hours, ensuring continued attempts to reach temporarily unavailable mailboxes.
- Adherence to Temporary Codes: Processing temporary error codes, like 4xx, correctly is paramount for ESPs to differentiate between transient issues and permanent non-delivery, guiding their retry efforts.
- Protecting User Subscriptions: To preserve subscriber lists and maintain deliverability, ESPs should avoid unsubscribing users or converting temporary bounces to hard bounces during temporary ISP outages.
Expert view
Expert from Spam Resource explains that during an ISP outage, such as with Yahoo or AOL, email service providers (ESPs) should anticipate a high volume of temporary bounces. It is crucial for ESPs not to immediately categorize these as permanent failures. Instead, they should hold onto the messages and continuously retry delivery for an extended period, typically around 72 hours, classifying these as soft bounces until the ISP systems recover.
28 Mar 2023 - Spam Resource
Expert view
Expert from Word to the Wise shares that when mailboxes are temporarily unavailable due to ISP outages or system issues, email service providers (ESPs) must queue the messages and diligently retry delivery based on temporary error codes, like 4xx. ESPs should persist in retrying messages for the standard duration, often up to 72 hours, for these temporary failures, rather than prematurely treating them as permanent bounces or unsubscribing users.
5 Nov 2023 - Word to the Wise
What the documentation says
8 technical articles
Email Service Providers (ESPs) consistently manage deliverability during outages at major providers like Yahoo or AOL by classifying these events as temporary interruptions, commonly known as 'soft bounces.' Across the industry, their systems are engineered to automatically defer and then repeatedly attempt delivery of affected messages over a defined period, frequently up to 72 hours. This standardized approach ensures that emails are eventually delivered once the recipient servers become available again, preventing immediate permanent bounce classifications and preserving ongoing email campaigns.
Key findings
- Universal Soft Bounce Handling: All surveyed ESPs categorize delivery failures due to temporary ISP outages as 'soft bounces,' indicating a transient issue rather than a permanent one.
- Standardized Retry Duration: A common industry practice among ESPs is to retry email delivery for approximately 72 hours during a recipient server outage, demonstrating a consistent strategy for managing temporary unavailability.
- Automated Retries: ESPs' systems are designed for automatic, persistent re-attempts at delivery, queuing messages and continuously trying to send them until the receiving server is restored or the retry window expires.
- Eventual Hard Bounce Conversion: If delivery cannot be completed within the specified retry period, the temporary soft bounce will eventually convert to a permanent 'hard bounce,' indicating an enduring inability to deliver.
- SMTP Error Code Interpretation: ESPs interpret specific SMTP transient error codes, such as 4xx series errors (e.g., 421 Service Not Available), to identify temporary issues and trigger appropriate retry logic.
Key considerations
- Accurate Bounce Categorization: It is paramount for ESPs to correctly identify temporary server outages as soft bounces, avoiding premature classification as permanent failures to maintain deliverability and sender reputation.
- Robust Retry Mechanisms: Implementing automatic, multi-attempt retry systems is essential to ensure that emails are not lost during transient ISP outages but are instead held and re-sent until the issue is resolved.
- Defined Retry Windows: Establishing and communicating a clear retry duration, typically up to 72 hours, helps manage expectations and ensures consistent handling of temporarily undeliverable messages.
- Strategic Hard Bounce Transition: ESPs must have a defined process for when and how soft bounces, after exhaustive retries, are converted into hard bounces, ensuring list hygiene without sacrificing deliverability during temporary outages.
- Preserving Subscriber Lists: The retry strategy prevents emails from being immediately marked as undeliverable or subscribers being unnecessarily removed from lists during temporary service disruptions.
Technical article
Documentation from SendGrid explains that ESPs classify temporary failures, such as those caused by an ISP outage, as 'soft bounces.' During such events, SendGrid's systems will temporarily defer sending and automatically retry delivery over a set period. If the issue resolves, the email will be delivered; otherwise, it may eventually become a hard bounce if the problem persists beyond the retry window.
1 Sep 2023 - SendGrid
Technical article
Documentation from Mailchimp explains that when an email server, like Yahoo or AOL, is temporarily unavailable due to an outage, Mailchimp categorizes these delivery failures as 'soft bounces.' Their system will continue to attempt delivery for a certain period, usually 72 hours, before marking the address as a permanent bounce if the issue isn't resolved.
24 Jun 2024 - Mailchimp
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