Quick Answer
Pause or reduce sending, identify whether bounces are hard or soft, clean the list, verify SPF, DKIM, and DMARC, check mailbox reputation, then restart with lower volume only after bounce patterns improve.
First Triage
Hard bounces usually mean the recipient address is invalid. Soft bounces can point to temporary blocks, rate limits, inbox issues, or recipient-side policy. Repeated Gmail or Outlook deferrals should be treated as a reputation warning.
Fix Order
Stop sending to the current list, export bounce messages and status codes, remove invalid addresses, check authentication, identify the domain or inbox causing most bounces, lower daily volume, and restart with smaller batches.
ScaledInboxes View
Infrastructure can only protect you so far. Bad lists burn good inboxes, so list quality belongs inside the deliverability operating system.