How do you handle ambiguous bounces (e.g., “undeliverable”)?;

Ambiguous bounces lack clear classification signals:

Common ambiguous messages:

"Message undeliverable." "Delivery failed." "Unable to deliver." Generic errors with no code.

Analysis strategies:

Check SMTP code if available. Examine full headers for context. Look for enhanced status codes. Check provider reputation patterns.

Conservative handling:

Default to soft bounce classification. Allow retry before suppression. Track for pattern emergence. Manual review for high-volume ambiguity.

Context clues:

Provider known for specific behaviors. Recent successful delivery to same domain. Domain verification status. Historical pattern for this address.

Escalation path:

Ambiguous > soft bounce > threshold tracking > hard bounce (if persistent).

When in doubt, err toward preservation. You can always suppress later; you cannot easily unsuppress.