Soft Bounce ​

A soft bounce is a temporary email delivery failure that occurs when an email cannot be delivered due to transient issues that may resolve over time. Unlike hard bounces which are permanent failures requiring immediate list removal, soft bounces represent provisional delivery obstacles that often warrant retry attempts.

What Causes Soft Bounces? ​

Soft bounces occur for several specific reasons related to temporary delivery obstacles:

• Full mailbox: The recipient's mailbox has reached storage capacity limits
• Server availability: The recipient's email server is temporarily down or not responding
• Message size limits: The email exceeds the recipient server's maximum size restrictions
• Greylisting: The receiving server temporarily rejects mail from unknown senders as an anti-spam measure
• Rate limiting: The receiving domain is throttling incoming connections from your sending IP
• Content filtering: Temporary deferral due to content scanning or processing delays
• DNS issues: Temporary DNS resolution problems affecting mail delivery

Each of these causes results in a temporary failure notification, typically accompanied by a 4XX SMTP error code. For example, a 421 error with a message like "Service not available" or a 450 "Mailbox unavailable" indicates a soft bounce condition.

Why Soft Bounces Matter ​

Soft bounces are important indicators for email marketers because they provide early warning signals of potential deliverability issues that could escalate if not monitored. While isolated soft bounces rarely impact sender reputation, patterns of persistent soft bounces to specific domains may indicate emerging problems with your sending practices or infrastructure. According to this AWS re:Post page, monitoring soft bounce subtypes (such as MailboxFull, MessageTooLarge, or ContentRejected) helps identify specific delivery issues that may require different handling approaches.

Unlike hard bounces, soft bounces don't typically require immediate list removal, but they do represent valuable engagement insights. For example, consistently full mailboxes often indicate inactive or abandoned accounts, suggesting these users are no longer engaging with email. Tracking soft bounce patterns across time can reveal technical delivery patterns such as server maintenance schedules or peak traffic periods at specific recipient domains, allowing for more strategic sending.

Soft Bounce Error Codes ​

Soft bounces are typically identified by their SMTP response codes in the 4XX range, indicating temporary failures:

These 4XX codes specifically indicate that the server has encountered a temporary issue, and retry attempts may succeed at a later time.

Handling Soft Bounces ​

Soft bounces require a strategic approach that balances patience with monitoring. Most email systems, including AWS SES, implement automatic retry logic, attempting delivery multiple times over a period before considering the delivery permanently failed. As noted in AWS documentation, SES categorizes soft bounces as "Transient" bounces with various subtypes, and only includes them in bounce metrics when SES is no longer retrying delivery. This accommodation period recognizes the temporary nature of soft bounces and allows recipient servers time to resolve transient issues. However, implementing persistence thresholds for addresses that repeatedly soft bounce is equally important, as recurring patterns may indicate dormant accounts.

Professional email senders should develop segmentation strategies for different soft bounce types, as each may warrant different handling. For example, full mailboxes suggest list inactivity and might prompt re-engagement campaigns, while server-related soft bounces typically require only patience. When monitoring soft bounces across campaigns, watch for domain-specific patterns that could reveal reputation or technical issues with specific mailbox providers, allowing for targeted remediation before deliverability is significantly impacted.

Frequently Asked Questions About Soft Bounces ​

How are soft bounces different from hard bounces? ​

Hard bounces represent permanent, unresolvable delivery failures (like non-existent addresses), while soft bounces are temporary issues (like full mailboxes) that may resolve with time. Hard bounces require immediate list removal, whereas soft bounces typically trigger retry attempts.

How many retry attempts should be made for soft bounces? ​

Industry standard practice is to attempt delivery between 3-5 times over a 24-72 hour period. After this window, continued attempts have diminishing returns and may waste sending resources.

When should I remove soft bouncing addresses from my list? ​

Consider removing addresses that consistently soft bounce across 5+ consecutive sending attempts or over a 30+ day period, particularly for "mailbox full" errors which suggest account abandonment. For server-related soft bounces, more patience may be warranted.

Do soft bounces affect sender reputation? ​

Individual soft bounces have minimal impact on sender reputation as mailbox providers recognize they represent temporary conditions. However, repeatedly sending to addresses that consistently soft bounce without implementing suppression rules may eventually impact engagement metrics.

How can I reduce soft bounce rates? ​

To minimize soft bounce rates, segment your lists by engagement level, respect recipient server limitations, avoid unnecessarily large attachments, implement proper authentication (SPF, DKIM, DMARC), and monitor your sending infrastructure for performance issues that could contribute to higher soft bounce rates.

Related Content ​

• Bounces
• Hard Bounce
• Bounce Rate
• SMTP
• Email Authentication
• Amazon SNS (Simple Notification Service)