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.