How do they decide when to suppress an address?

Suppression decisions balance accuracy against list protection:

Immediate suppression triggers:

Hard bounces: User not found, domain invalid. Explicit permanent codes: 550, 5.1.1. Account disabled: Clear permanent status.

Threshold-based suppression:

Soft bounces: Suppress after N consecutive failures (typically 3-7). Time window: Failures over 7-30 days. Pattern analysis: Consistent failure reason.

Delayed suppression factors:

Block bounces: May be temporary reputation issues. Ambiguous codes: Require investigation. Recent engagement: May indicate false positive.

Suppression levels:

Permanent: Never attempt again. Temporary: Retry after cooling period. Campaign-specific: Suppress for current send only.

Suppression decisions protect reputation while preserving valid subscribers. Too aggressive loses subscribers; too lenient damages reputation.