How to interpret ESP bounce classifications?

ESPs categorize bounces to help you understand why delivery failed and what action to take. Classifications vary by ESP but follow common patterns.

• Hard bounces (permanent failures, remove immediately):
• User unknown: Address doesn't exist
• Domain not found: Domain doesn't exist or has no MX
• Bad address syntax: Malformed email address
• Policy rejection: Permanently blocked by recipient policy
• Soft bounces (temporary failures, monitor and retry):
• Mailbox full: Recipient's mailbox over quota
• Server temporarily unavailable: Receiving server down
• Message too large: Size exceeds recipient limits
• Deferral/throttling: Rate limited, will retry
• Content filtering: Temporary block due to content
• Important distinctions:
• Repeated soft bounces may indicate inactive addresses (treat as hard after N failures)
• Some blocks classified as soft are really reputation issues
• ESPs may have proprietary categories beyond standard hard/soft
• What to do:
• Hard bounces: Suppress immediately, never send again
• Soft bounces: Retry per ESP policy; suppress after persistent failures
• Review bounce reports to spot patterns (domain issues, list quality, content triggers)
• Understand your ESP's specific classification logic through their documentation.