How to tell if soft bounces were eventually delivered?

Key findings

• ESP logging differences: The visibility of whether a soft bounce eventually delivered heavily depends on the ESP's logging practices. Some ESPs might only report a soft bounce after multiple retries have failed and the email is abandoned, while others might provide more granular data on transient failures and subsequent successful deliveries.
• VERP for tracking: Using Variable Envelope Return Path (VERP) can assist in cross-referencing delivered events with prior soft bounces within your own system logs. This method assigns a unique return path to each recipient, making it easier to track individual message statuses.
• Ambiguity in definition: There is no universal standard for defining soft bounces. Some ESPs use it broadly for any non-hard bounce, while others distinguish between deferrals (temporary delays with retries) and specific soft bounce reasons like a full mailbox.
• Common soft bounce codes: SMTP error codes like 421 (service not available, server busy) and 451 (account under maintenance) are typical indicators of temporary issues that might resolve themselves with retries.

Key considerations

• Review ESP analytics: Check if your ESP provides detailed delivery logs that show not just the final bounce status, but also any preceding deferred states or successful retries. This can offer insight into whether a message initially delayed eventually delivered.
• Look for subsequent engagement: For emails that soft bounced, monitor for opens, clicks, or other user engagement metrics from that recipient. If these occur, it's a strong indication the email was eventually delivered, even if it initially experienced a temporary setback.
• Understand ESP retry policies: Familiarize yourself with your ESP's internal retry mechanisms. Many ESPs will attempt to redeliver messages that incur transient (soft bounce) errors for a certain period before giving up and classifying it as a permanent soft bounce. More information can be found on Shopify's guide on bounce types.
• Distinguish between deferrals and soft bounces: While often conflated, a true deferral signals the recipient server asking the sender to try again later, whereas a soft bounce (like a full mailbox) means the server accepted it but couldn't deliver it right then. Knowing this distinction helps in tracking. You can learn more about how to handle 4xx mail errors for better deliverability.

Key opinions

• ESP-specific handling: Many marketers believe that how soft bounces are managed, including retries and eventual delivery, is largely dependent on the individual ESP's internal policies and logging capabilities. ESPs classify and manage bounce codes differently.
• No universal standard: There isn't a universally accepted definition for a soft bounce, leading to varying interpretations among marketers and providers. Some consider delays as soft bounces, while others differentiate them as deferred errors.
• Engagement as an indicator: Marketers often rely on subsequent opens and clicks as clear indicators that a message, even if initially flagged as a soft bounce or delay, eventually reached the inbox.
• Data issues impacting bounces: Sometimes, recurring soft bounces, even with temporary error codes like 4xx, can signal underlying data quality issues rather than transient server problems, suggesting the need for list hygiene.

Key considerations

• Clarify with your ESP: The most effective way to know if soft bounces convert to deliveries is to ask your ESP directly what data they make available. They hold the definitive logs for delivery attempts and outcomes.
• Leverage internal logs: If your ESP doesn't provide granular data, check your own campaign logs for successful delivery events to recipients who previously soft bounced. This indicates a successful retry.
• Monitor engagement metrics: Track opens, clicks, and conversions for recipients who experienced soft bounces. Positive engagement confirms delivery, regardless of the initial bounce status. More on soft bounces and their resolution can be found via Mailgun.
• Understand retry windows: Be aware that an email reported as a soft bounce generally means the ESP has exhausted its retry attempts for that specific send. Subsequent successful deliveries would likely occur on a new mailing. Consider how soft bounce retry policies affect domain reputation.

Key opinions

• Log analysis is key: Experts advise examining detailed delivery logs for 'delivered' events that correspond to prior soft bounces. This requires robust logging capabilities either by the sender or the ESP.
• ESP logging opacity: The way ESPs handle and report bounces, including the level of detail provided to senders, can be opaque. This often makes it difficult for senders to get clear answers on whether a soft bounce was eventually delivered.
• Delay vs. soft bounce: A 'delay' (or deferred status) often precedes a soft bounce and indicates that the ESP is actively retrying delivery. A reported soft bounce might signify that the ESP has abandoned retries for that particular message.
• No universal definition: There's no industry-wide agreement on what constitutes a soft bounce. This lack of standardization contributes to confusion among senders regarding true delivery status.

Key considerations

• Utilize VERP: Implementing VERP can simplify cross-referencing, as it provides a unique identifier for each bounced email, making it easier to match it with a subsequent delivered event if retries are successful. More on VERP's utility in bounce management can be found from Twilio's blog on email bounce management.
• Engage with your ESP: Direct communication with your ESP is often necessary to understand their specific bounce classification and logging practices, and to request access to more granular data.
• Cross-reference with recipient data: If the ESP doesn't provide explicit delivery confirmations after a soft bounce, look for successful delivery attempts to the same recipient within the same campaign using your own internal records. This confirms a retry resulted in delivery.
• Monitor broader reputation: While individual soft bounces may resolve, a consistently high soft bounce rate or blocklist listing can negatively affect your sender reputation and inbox placement for future campaigns.

Key findings

• SMTP 4xx codes: The SMTP specification (e.g., RFC 5321) categorizes 4xx reply codes as 'transient negative completion' replies. These indicate a temporary failure, implying that the sending MTA should retry delivery after some time.
• Implied retries: When a 4xx error is received, the expectation is that the sending server will attempt to redeliver the message later. The mail is queued for retry, and if the issue resolves, it can be delivered successfully without the sender explicitly knowing it was ever 'bounced'.
• Retry policy variability: While retries are standard, the exact number and timing of these attempts are not strictly defined by core RFCs. This allows ESPs and mail servers flexibility in implementing their retry logic before abandoning a message and declaring a final soft bounce.
• Delivery confirmation: Ultimately, a 2xx SMTP reply code (e.g., 250 OK) from the recipient's mail server is the definitive confirmation of successful acceptance, regardless of any prior temporary failures.

Key considerations

• RFC compliance: Modern ESPs and MTAs generally adhere to RFCs concerning temporary errors, meaning most transient failures will trigger retries. This is a fundamental aspect of email reliability.
• Understanding 4xx codes: Familiarize yourself with common 4xx SMTP bounce codes and their meanings to interpret bounce notifications accurately and distinguish between temporary and permanent issues.
• Focus on the end result: The primary goal is successful delivery. If your ESP confirms delivery or you observe engagement, the initial soft bounce notification (or deferred status) becomes less critical as the message reached its destination. For more technical insight, refer to RFC 5321, the internet message format standard.