How do I identify if the issue is list hygiene, engagement, or technical?

Technical issues:

• Authentication failures in headers
• Affects all mail equally
• Often sudden onset
• DNS or configuration changes correlate

Engagement issues:

• Gradual decline over time
• Worse performance on older segments
• Better performance on recent signups
• No sudden trigger event

List hygiene issues:

• Bounce rate spikes
• Complaint rate increases
• Correlates with new list additions or reactivation
• Spam trap indicators

Diagnosis approach: Check authentication first (fast), then correlate timing with list/campaign changes, then analyze engagement patterns.

Is the ship broken, is the cargo spoiled, or have the trade routes dried up?