My deliverability dropped suddenly, how do I find the cause?

Immediate checks:

• Authentication: Are SPF, DKIM, DMARC passing? Any DNS changes?
• Blocklists: Check MXToolbox, Spamhaus for IP and domain.
• Postmaster: Gmail reputation, SNDS status.

Data analysis:

• Bounces: New error codes or patterns?
• Complaints: Spike in feedback loop reports?
• Volume: Recent increases or unusual patterns?

Scope identification:

• All providers or specific ones?
• All campaigns or specific types?
• All segments or specific audiences?

Timeline correlation:

• What changed in the 48-72 hours before the drop?
• List changes, content changes, infrastructure changes?
• Document findings as you investigate.

Methodical investigation: was it the ship, the cargo, or the destination that changed?