What are the most common causes of sudden deliverability drops?

Frequent causes of sudden deliverability drops:

Blocklist listing: IP or domain added to major blocklist. Immediate, widespread impact.

Complaint spike: Sudden increase in spam reports triggers filtering.

Authentication failure: SPF, DKIM, or DMARC breaks due to DNS or configuration changes.

List quality event: Sending to old list, purchased list, or spam trap.

Volume spike: Sudden increase triggers rate limiting or suspicion.

Content trigger: New content pattern matches spam signatures.

Provider algorithm change: Gmail or Microsoft updates filtering. Affects many senders.

Diagnosis starts with identifying which of these occurred, often through process of elimination.

Sudden harbor restrictions can result from cargo problems, ship issues, or port policy changes.