What causes delivery errors when no hard bounces are seen in the ESP?

Key findings

• Deferrals: The most common cause for delivery errors without hard bounces is a temporary deferral by the recipient's server. These are often indicated by a 4xx SMTP error code.
• Retry periods: ESPs typically retry sending deferred emails for a set period, such as 48 hours. During this time, the mail is still in a queue and might eventually be delivered or turn into a hard bounce if retries fail permanently.
• Spam filtering impact: Sending to the bulk folder or encountering spam filters can trigger temporary rejections, contributing to these unclassified delivery errors.
• Mailbox full: A common reason for a deferral (specifically a 421 or 452 error) is the recipient's mailbox being full or temporarily unavailable.
• Gmail accuracy: For Gmail contacts, delivery errors often closely match the percentage of deferral or transient errors in senders' logs.

Key considerations

• Review ESP logs: Dive deeper into your ESP's logs to identify specific deferral reasons and SMTP error codes, which can provide more granular insights than summarized delivery error reports.
• Understand retry policies: Familiarize yourself with how long your ESP attempts to retry deferred emails, as this directly impacts when (or if) these messages will ultimately be delivered.
• Monitor inbox placement: Even with deferrals, emails may eventually land in the spam folder, not the inbox. Regularly test your inbox placement to catch these issues.
• Isolate problematic sends: If a specific campaign correlates with a spike in delivery errors, analyze that message and its audience for potential triggers for deferrals or spam classification. This can include checking if your emails are going to spam.
• Differentiate bounces: Understand the difference between hard and soft bounces, as discussed in detail by SendLayer's guide to hard versus soft bounces.

Key opinions

• Deferral likelihood: Many marketers suspect these uncategorized delivery errors are actually deferrals, where the email is temporarily rejected but remains in the sending queue for retries.
• Correlation with specific sends: Some find that these errors correlate with specific email campaigns, suggesting content, list quality, or sending patterns might be a factor.
• Gmail's role: A common observation is that a high percentage of these errors occur with Gmail or Google Mail contacts, pointing to specific handling by Google's servers.
• Spam folder routing: Marketers note that if emails are heavily routed to the bulk (spam) folder, it can trigger temporary errors, which might not be explicitly reported as a bounce.
• Eventual delivery: There's an understanding that temporary failures, or 'tempfails,' can ultimately lead to successful delivery after subsequent attempts.

Key considerations

• Check ESP configurations: Confirm how your ESP defines and reports different types of delivery errors, especially transient ones, as reporting can vary between platforms.
• Analyze deferral logs: Focus on specific deferral reasons in detailed logs for providers like Gmail, looking for trends such as an increase in 'mailbox full' errors.
• Segment problematic campaigns: If a specific campaign is causing the spike, investigate its content and the targeted audience. This might reveal issues like undelivered B2B client emails.
• List hygiene importance: Even if not hard bouncing, persistent soft bounces or deferrals can indicate list quality issues that need addressing to prevent future deliverability problems. Consider the hidden factors affecting your email deliverability rate.
• Impact of soft bounces: Recognize that soft bounces, while temporary, can still damage marketing efforts and sender reputation, as highlighted by SocketLabs on email marketing red flags.

Key opinions

• Deferral identification: Experts commonly identify deferrals (temporary failures) as the primary reason for delivery errors not registering as hard bounces.
• Retry mechanism: The email might still be in the ESP's queue, awaiting retries. The ultimate outcome, whether successful delivery or a permanent failure, depends on the ESP's retry duration.
• SMTP 4xx codes: These errors often correspond to 4xx SMTP response codes, which signal temporary conditions (e.g., recipient server busy, mailbox full, greylisting) that may clear up with time.
• Spam placement trigger: A significant number of emails being sent to the spam folder can trigger 421 deferrals or similar temporary blocks from the receiving mail server.
• Reputation impact: Even temporary failures and soft bounces can negatively affect sender reputation over time, potentially leading to more frequent deferrals or blocklisting (or blacklisting).

Key considerations

• Analyze SMTP responses: Go beyond aggregated ESP reports and examine raw SMTP logs for 4xx series errors to understand the exact nature of the temporary failure. This helps diagnose issues like Gmail soft bounce errors.
• Sender reputation monitoring: High volumes of deferrals can be an early indicator of a declining sender reputation. Proactive monitoring of your domain and IP reputation is crucial.
• Greylisting awareness: Understand that greylisting can cause initial deferrals as a spam prevention technique, requiring the ESP to retry the send.
• Content and list quality: While not a hard bounce, consistent deferrals may signal issues with email content triggering spam filters, or a list that contains many inactive or problematic addresses.
• Distinguish from hard bounces: It's critical to understand that these temporary issues are distinct from hard bounces, which signify permanent, unfixable delivery failures, as detailed in Mailgun's guide to hard bounces.

Key findings

• SMTP error codes: The SMTP protocol uses 4xx series reply codes (e.g., 421, 451, 452) to signify transient negative completion replies, indicating temporary failures that might succeed on retry.
• Soft bounce definition: Documentation defines soft bounces as temporary delivery failures due to issues like a full mailbox, a temporarily unavailable server, or a message exceeding size limits.
• Distinction from hard bounces: Hard bounces are distinguished as permanent failures, typically due to invalid recipient addresses or non-existent domains, which receive 5xx SMTP reply codes.
• Retry mechanisms: ESPs are expected to implement retry logic for deferred emails, attempting delivery multiple times over a defined period before giving up.
• Greylisting: Some receiving servers employ greylisting, which temporarily rejects emails from unknown senders, requiring a retry to accept the message, thus causing a deferral.

Key considerations

• Adherence to RFCs: Senders should ensure their ESP adheres to RFC standards for SMTP response codes to accurately interpret delivery failures, including transient errors. Understanding what RFC 5322 says is a good start.
• ESP reporting transparency: Look for ESPs that provide transparent reporting on deferrals and soft bounces, distinguishing them from hard bounces to offer a clearer picture of deliverability. Some ESPs have more deliverability service gripes.
• Policy-based rejections: Be aware that receiving mail servers might issue temporary rejections based on their anti-spam policies or temporary trust issues, requiring multiple attempts before acceptance.
• Understanding bounce types: Refer to documentation from authoritative sources like Klaviyo's guide to bounced emails to clearly differentiate between hard and soft bounces.