What data sources should I check first? (e.g., bounces, complaints, Postmaster Tools)

Data source priority for diagnosis:

1. Postmaster Tools (Gmail, SNDS)

Current reputation status

Authentication pass rates

Spam rate data

2. Bounce logs

Error codes reveal block reasons

Pattern changes indicate new problems

3. Complaint data

Feedback loop reports

Timing and source of complaints

4. Blocklist checks

MXToolbox, Spamhaus, Barracuda

IP and domain separately

5. Engagement metrics

Opens, clicks by provider

Changes over time

6. Sending logs

Volume changes

List composition changes

Start with reputation indicators, then dive into specifics.

Check the harbor status board first, then inspect the cargo manifest.