What are common email bounce messages and what do they mean?

Key findings

• Diverse causes: Bounces can stem from a variety of issues, including non-existent email addresses, full mailboxes, network problems, or recipient server blocks.
• SMTP codes: Specific SMTP (Simple Mail Transfer Protocol) codes, such as 550 or 554, provide detailed reasons for delivery failures, which are essential for diagnosis. You can find more information on common bounce codes on our site.
• New domain challenges: Newer domains, especially those under 30 days old, often face higher bounce rates due to lack of established sender reputation.
• Content impact: Even elements like excessive backlinks, emojis, or bolding in email content can contribute to messages being blocked or bounced by recipient servers.

Key considerations

• Identify specific bounce types: Merely knowing you have soft or hard bounces is not enough; obtaining the exact SMTP bounce messages is critical for effective troubleshooting. As Mailgun points out, understanding the specific error codes helps you react appropriately.
• Warm-up strategy: For new domains, a proper warm-up strategy, gradually increasing sending volume to engaged recipients, is essential to build sender reputation and reduce bounce rates.
• List hygiene: Implement double opt-in and regularly segment or clean lists to remove invalid or unengaged addresses that contribute to hard bounces. This minimizes their impact on your deliverability.
• Content review: Periodically audit email content for elements that might trigger spam filters, such as excessive formatting, suspicious links, or certain emoji usage, especially if you are experiencing unusually high bounce rates for otherwise legitimate emails.

Key opinions

• Proactive segmentation: Marketers frequently emphasize creating segments to exclude recipients who have previously bounced, preventing re-attempts to invalid addresses.
• Content scrutiny: There's a strong belief that certain content elements, like numerous backlinks, excessive bolding, or emojis in subject lines, can negatively impact deliverability and lead to bounces.
• Domain age impact: New domains are perceived to be at a disadvantage, often experiencing higher bounce rates until sender reputation is established.
• Warm-up limitations: Relying solely on engagement flows like welcome series for warming up a new domain may not be sufficient to mitigate initial high bounce rates across all campaigns, particularly for transactional or automated emails. Read our guide on improving domain reputation.

Key considerations

• Identify bounce root causes: It is crucial to look beyond the generic classifications of soft and hard bounces and investigate the specific bounce messages returned by the receiving servers to pinpoint exact issues.
• Strategic sending volume: Even for low-volume sending (e.g., less than 50 emails daily), high bounce rates on critical flows like abandoned checkout or thank-you emails indicate underlying problems that need immediate attention.
• Dedicated sending domain: Consider if a dedicated sending domain is necessary to better control your sending reputation, especially if using a shared IP and experiencing persistent bounce issues. This can significantly impact your overall email deliverability.
• List acquisition methods: Review email collection methods (e.g., sign-up forms, Shopify footer/checkout) to ensure quality and consent, as poor list hygiene is a leading cause of bounces.

Key opinions

• Bounce messages are paramount: Experts universally agree that the terms "soft" and "hard" bounces are insufficient without the precise bounce message or SMTP code, as these provide the real diagnostic information.
• Volume and reputation: Sending volume, particularly for new domains, directly impacts how receiving mail servers perceive your mail, often leading to bounces if reputation is not carefully built.
• Content nuances: Beyond obvious spam, subtle content elements can trigger spam filters and result in bounces, such as too many links or certain formatting choices.
• Sender behavior matters: Receiving servers often block emails not just for invalid addresses, but also based on the sender's past behavior, including their IP address reputation or if they appear on a blocklist.

Key considerations

• Prioritize specific bounce codes: When facing high bounce rates, the first step is to obtain and analyze the specific SMTP codes returned. Generic problems like 550 5.1.1 (user unknown) require different actions than, for instance, a 554 (message refused) code.
• Assess sender reputation: For new domains or sudden bounce spikes, thoroughly review your sender reputation. Factors like recent inclusion on a blocklist or a rapid increase in sending volume without proper warm-up can cause receiving servers to block your mail.
• Implement robust authentication: Ensure your domain has correctly configured SPF, DKIM, and DMARC records. Misconfigurations can lead to authentication failures and bounce messages like 550 5.7.1 relay access denied, indicating policy rejections.
• Monitor and adapt: Continuously monitor bounce logs and adapt sending practices. Even after initial fixes, new issues can arise, necessitating ongoing attention to email hygiene and content practices.

Key findings

• RFC compliance: Bounce messages adhere to specific RFCs, such as RFC 3463 for Delivery Status Notifications, which define the structure and meaning of status codes (e.g., 5.1.1 for bad destination mailbox address).
• Permanent failures (5xx): Codes in the 5xx series indicate permanent fatal errors, meaning the email will never be delivered to that recipient. Examples include 550 (mailbox not found), 554 (transaction failed), or 571 (policy reasons).
• Temporary failures (4xx): Codes in the 4xx series denote temporary transient errors, suggesting the email might be delivered later if re-sent. Examples include 450 (mailbox unavailable, e.g., mailbox full) or 421 (service not available).
• Specific sub-codes: Further detail is provided by sub-codes (e.g., 5.1.1, 5.7.1), which pinpoint the exact reason for the bounce, such as no MX record found for a domain.

Key considerations

• Automated processing: Email sending systems should be configured to automatically parse and act on bounce messages, removing hard bounces immediately to protect sender reputation and retrying soft bounces according to best practices.
• Policy-based rejections: Many 5xx bounces, particularly those with 5.7.x sub-codes, are due to recipient server policies (e.g., anti-spam, authentication failures). Senders must ensure their SPF, DKIM, and DMARC records are correctly implemented.
• Throttling and rate limits: Receiving mail servers often implement rate limiting or throttling, leading to temporary bounces. Documentation advises senders to respect these limits and gradually increase sending volume to avoid being blocked. ActiveCampaign provides a useful glossary of bounce types.
• Content and reputation: Documentation also highlights that content quality and sender reputation play a significant role in determining whether an email is accepted or bounced, even if the address is valid. Avoiding spammy triggers is always recommended.