Summary
It can be perplexing when email users you know are deliverable start receiving hard bounces, especially since hard bounces are typically defined as permanent failures. This paradox often stems from a combination of factors, including temporary server issues, inaccurate bounce classifications by email service providers (ESPs), and nuances in how different mailbox providers handle email rejections. While a hard bounce usually implies an address is permanently invalid, there are indeed scenarios where a supposedly undeliverable email address becomes deliverable again, highlighting the complexity of email deliverability.
Key findings
- ESP definitions: Email service providers often categorize bounces as hard or soft based on their internal rules, which may not always align with the actual SMTP rejection codes.
- Temporary blocks: Sometimes, emails are temporarily blocked due to spam filtering or blocklist (or blacklist) entries, leading to a hard bounce classification that is not permanent.
- Mailbox provider glitches: Major mailbox providers (like Yahoo or Gmail) can experience system outages or temporary issues that result in what appear to be permanent rejections.
- Inaccurate classification: An ESP might incorrectly classify a temporary delivery failure as a hard bounce, suppressing a perfectly valid recipient.
- Smaller domains: Users with mailboxes on smaller or less reliable internet service providers (ISPs) may experience more frequent, temporary delivery issues that get classified as hard bounces.
Key considerations
- Review bounce logs: Always try to access the raw rejection messages from the recipient mailbox provider to understand the true reason for the bounce, beyond your ESP's classification.
- Assess ESP policies: Understand how your ESP defines and handles hard bounces. Some platforms are very aggressive in suppressing addresses after the first bounce.
- Identify patterns: Look for trends in false hard bounces, especially if they are coming from specific mailbox providers or smaller domains.
- Re-engagement strategy: If users confirm they should be receiving emails despite a hard bounce, have a process to re-enable them after careful investigation. For more, see what to do when an email hard bounces.
- Sender reputation: While legitimate hard bounces should be removed for sender reputation, overly aggressive suppression of valid addresses can harm user experience.
What email marketers say
Email marketers frequently encounter the confusing situation where contacts they know are valid suddenly receive hard bounces. This is particularly frustrating when the ESP's policies enforce immediate suppression, preventing further communication with these engaged users. Marketers highlight the need for greater flexibility and transparency from their ESPs to navigate these false positives effectively.
Key opinions
- Common occurrence: It is a common issue for deliverable users to sometimes receive hard bounces due to temporary problems or misclassifications.
- ESP control: Many ESPs, such as Responsys, have rigid policies that automatically remove users on the first hard bounce, limiting marketer control.
- Smaller domain impact: The problem of false hard bounces seems more prevalent with users who have smaller, purchased mailboxes rather than large providers like Gmail or Yahoo.
- Risk of removal: Marketers worry about preventing engaged, deliverable users from receiving emails if they are inaccurately marked as undeliverable.
- Seeking clarification: Marketers need a way to understand why these false hard bounces occur and how to ensure their emails reach intended recipients.
Key considerations
- ESP dialogue: Initiate discussions with your ESP to understand their bounce classification algorithms and explore options for less aggressive suppression of bounces, as discussed in ways to fix hard bounces.
- Segment by domain: Pay close attention to bounce rates from smaller domains, as they may behave differently than larger providers. For instance, Yahoo email addresses sometimes hard bounce despite validation.
- User communication: If users actively reach out claiming they are not receiving emails, prioritize investigating their specific bounce reason.
- Data insights: Use bounce data to identify recurring patterns or specific error codes (like USER_NOT_FOUND) that might indicate misclassification.
Marketer view
Marketer from Email Geeks notes that users known to be deliverable can indeed receive hard bounces, particularly when spam blocking is a factor. They observe that a block today might be resolved tomorrow, and even major mailbox providers like Yahoo and Gmail can experience temporary glitches that cause these issues.
Marketer view
Marketer from Email Geeks explains that their ESP (Responsys) considers it highly risky not to remove users after the first hard bounce. They feel limited by their ESP's algorithms, which prevent them from making changes to this process, despite knowing that some users expect and can receive these emails.
What the experts say
Deliverability experts clarify that the concepts of 'hard' and 'soft' bounces are often interpretations made by ESPs, not direct classifications from the recipient mailbox providers themselves. They emphasize that while many 5xx SMTP rejection codes do indicate permanent issues, temporary network or server problems can sometimes trigger responses that ESPs mistakenly categorize as hard bounces. Accessing the precise rejection message from the receiving server is paramount for accurate diagnosis.
Key opinions
- ESP classification: Experts stress that 'soft bounces' and 'hard bounces' are terms ESPs use for reporting, not inherent email concepts.
- False positives: It is possible for ESPs to misclassify rejections; a 5xx code doesn't always mean an address is permanently undeliverable.
- Aggressive removal: Removing recipients after the very first 5xx rejection is often considered too aggressive due to potential false positives.
- Specific rejection message: The only way to truly understand a bounce is to see the exact rejection message from the recipient's mailbox provider.
- ISP reliability: Smaller or 'mom & pop' ISPs might be less reliable, leading to occasional, temporary bounces that are miscategorized. For more, see increase in hard bounces from smaller domains.
Key considerations
- Demand access: Insist that your ESP provides you with the raw rejection messages from the receiving mail servers.
- Evaluate ESP accuracy: Understand that ESP bounce classifications may be more accurate for large providers than for smaller ones.
- Consult experts: If your ESP is inflexible, consider seeking expert advice on how to craft an argument for policy changes, especially when it's business-impacting.
- Monitor blocklists: Being on a shared blocklist (like Spamcop or Mimecast) can lead to temporary rejections that might be misclassified as hard bounces. Refer to email undeliverability reasons from Kickbox.
- Diagnose USER_NOT_FOUND: While this error usually suggests a permanent state, if you know the user exists, it indicates a misclassification requiring deeper investigation.
Expert view
Expert from Email Geeks suggests always looking at the original rejection message from the ESP when a hard bounce occurs. This raw data is critical for understanding the true reason behind the bounce.
Expert view
Expert from Email Geeks states that there is no inherent concept of 'soft' or 'hard' bounces in email protocols; these are terms coined by ESPs to simplify their rules. This means an ESP's 'hard bounce' declaration is their interpretation, not necessarily the mailbox provider's final word.
What the documentation says
Technical documentation and research consistently define hard bounces as permanent delivery failures, often due to invalid addresses or non-existent domains. However, they also acknowledge that temporary server issues, full mailboxes, or content-related filtering can lead to rejections that might be misinterpreted as permanent. The consensus emphasizes understanding the underlying SMTP codes and the potential for transient conditions to mimic permanent failures.
Key findings
- Permanent failure: A hard bounce signifies that an email cannot be delivered due to an unresolvable issue, like an invalid address or closed account.
- Temporary issues: Soft bounces are caused by temporary problems such as a full inbox, server downtime, or a message being too large.
- SMTP codes: Understanding specific SMTP 5xx codes (permanent errors) and 4xx codes (temporary errors) is crucial for accurate bounce handling.
- Misspelled addresses: Typos in email addresses are a common cause of hard bounces.
- Server rejection: When the recipient's email server rejects a message, it generates a bounce notification back to the sender.
Key considerations
- List hygiene: Regularly clean email lists to remove invalid or non-existent addresses to reduce hard bounces and protect sender reputation.
- Bounce rate management: A high bounce rate can negatively impact deliverability, making it vital to distinguish between soft and hard bounces.
- Recipient server issues: Problems on the recipient's mail server, such as crashes, maintenance, or being overloaded, can cause undelivered emails.
- Automated suppression: While most ESPs automatically remove hard-bounced addresses, understanding the nuance of why email bounces happen is key.
Technical article
Documentation from Mailchimp defines a hard bounce as a permanent reason an email cannot be delivered. It typically leads to the removal of the email address from audience lists, as delivery attempts will consistently fail.
Technical article
Documentation from Kickbox Blog states that a hard bounce means an email could not be delivered due to a permanent failure. This failure prevents future delivery attempts to that specific address.
Related resources
13 resources
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