What causes soft bounces for a single contact and how should they be handled?

Key findings

• Temporary issues: Soft bounces usually stem from temporary problems that might resolve on their own, such as a full mailbox, a server being offline, or an oversized message. Mailchimp explains the difference.
• Configuration errors: Recipient server misconfigurations, like those on an on-premise Exchange server or O365 forwarding loops, can lead to sporadic soft bounces. These might block only some messages, based on content or other conditions.
• Inconsistent bounces: A single contact might experience soft bounces for some emails but not others, suggesting issues like content-based filtering or dynamic server load, rather than a permanently invalid address.
• Bounce code specifics: The specific SMTP error codes (e.g., 4xx codes) provide clues to the temporary nature of the issue. You can explore how email service providers manage these codes in more detail. Learn about bounce codes and tolerance.

Key considerations

• Individual assessment: When a single contact consistently soft bounces, it's wise to review the bounce reasons individually rather than treating it as a broad deliverability issue. This helps distinguish isolated problems from widespread ones.
• Suppression criteria: If a contact frequently soft bounces (e.g., 50-60% of sends) and shows no engagement (no opens), it's often prudent to stop sending to them. This helps maintain a healthy email list and protects sender reputation. Consider best practices for managing hard and soft bounces. Manage bounces in daily campaigns.
• Monitoring patterns: Pay attention to whether soft bounces are isolated to a single contact or are affecting an entire domain. This distinction guides the appropriate response, such as content adjustments versus broad suppression.
• Content impact: If only some emails to a contact soft bounce, investigate if specific content elements (e.g., links, images, keywords) in those emails might be triggering recipient-side content filters.

Key opinions

• Temporary but persistent: Many marketers note that while soft bounces are technically temporary, for a single contact, they can indicate a persistent problem at the recipient's end that might not resolve without intervention.
• Content-specific issues: If some emails bounce and others do not, marketers often suspect content-based filtering or dynamic server-side blocking based on message characteristics.
• Observation is key: It's important to monitor a single contact's bounce history to identify patterns before taking drastic action. This includes looking at bounce codes and specific error messages.
• Engagement matters: If a soft-bouncing contact also shows zero opens or clicks, it reinforces the decision to eventually suppress them, as they are not engaging with your content.

Key considerations

• Individual suppression: For individual contacts who frequently soft bounce and show no engagement, marketers generally agree on turning off further sends to them to avoid unnecessary bounces and protect domain reputation. Consider if it is bad to email addresses that keep soft bouncing. Is it bad to email them?
• Avoid immediate hard bounce classification: Do not immediately mark a soft bounce as a hard bounce. Soft bounces imply a temporary state; premature classification can lead to losing a valid contact. Understand hard and soft bounce differences.
• Content review: If a content filter is suspected, review the content of the bounced emails for potential triggers like suspicious links, certain keywords, or too many images. SendLayer advises on fixing bounces.
• Suppression logic: Implement a clear soft bounce suppression logic that accounts for the number of consecutive bounces and lack of engagement before deactivating a subscriber.

Key opinions

• Recipient-side configuration: Experts often attribute soft bounces to recipient-side configuration issues, which can be specific to a particular MX record or a customer's individual setup, like a misconfigured on-premise Exchange server. This explains why bounces might not occur across an entire domain.
• Mail loops and filters: Some soft bounces, especially from systems like O365, can be due to mail forwarding loops where an email is blocked by a downstream filter, then bounced back to the forwarding server, creating a loop.
• Content-based blocking: An expert opinion suggests that if only a subset of emails to a single contact soft bounces, content-based filters are likely culprits. This would explain selective blocking of messages.
• Treating Outlook loops: When an expert encounters mail loops from Outlook, they generally interpret it as a block occurring further down the delivery chain, not necessarily within O365 itself.

Key considerations

• Isolate the issue: Experts advise that if soft bounces are confined to a single contact, and not indicative of a wider domain or organizational issue, the problem should be addressed specifically for that contact. Learn why soft bounces happen.
• Evaluate persistence: When a contact consistently soft bounces (e.g., 50-60% of the time) and shows no engagement, experts generally recommend ceasing sends to them. This pragmatic approach prevents unnecessary resource waste and protects sender reputation. Determine soft bounce suppression limits.
• Interpret bounce messages carefully: SMTP error messages provide valuable clues but may not always tell the full story, especially with complex forwarding or filtering setups. Look beyond the initial error for deeper insights. Twilio offers email bounce management tips.
• Content optimization: If selective soft bounces occur, experts suggest reviewing email content that triggered the bounces. Adjusting content, reducing link density, or simplifying formatting might help bypass filters.

Key findings

• Transient failure codes: Documentation specifies that soft bounces are associated with 4xx SMTP status codes, meaning a temporary negative completion reply. This instructs the sender to retry later, implying that the issue might be resolved.
• Common temporary reasons: Typical reasons cited in documentation include recipient mailbox full, server unavailable, message too large, or exceeding the recipient's message limits.
• Differentiated from hard bounces: RFCs and other standards clearly distinguish soft bounces from hard bounces (5xx codes), which indicate permanent failure and no possibility of future delivery to that address. Read about hard versus soft bounces.
• Sender responsibility for retries: The SMTP protocol mandates that sending mail servers should retry soft-bounced messages for a period, typically several days, before giving up entirely. This is why a single contact might receive some, but not all, emails.

Key considerations

• Adherence to retry mechanisms: Email service providers (ESPs) should adhere to standard retry mechanisms for soft bounces. Their internal logic determines how many attempts are made and over what period. Recommended soft bounce suppression logic.
• Monitoring bounce codes: While 4xx codes are temporary, the specific sub-codes can offer more precise diagnostic information, helping to distinguish between a full mailbox and a server issue. For example, 4003 and 4004 Gmail soft bounce errors have specific causes. Resolve Gmail 4003 and 4004 errors.
• Impact on sender reputation: Although individual soft bounces are less damaging than hard bounces, a high volume of persistent soft bounces can still negatively affect sender reputation and inbox placement over time. Twilio discusses bounce management.
• Long-term suppression: If a soft bounce persists over an extended period or multiple retries without success, even official guidelines suggest eventually ceasing attempts, treating it as a functionally undeliverable address despite its technical 'temporary' status.