Start with when the complaint rate changed and which traffic changed with it.
Break the stream down by sending domain, campaign, list source, template, audience, and provider where data is available.
Then ask:
- Was a new list imported?
- Did send frequency jump?
- Did branding or From identity change?
- Is unsubscribe working?
- Was an old or inactive segment contacted?
- Did an account or sending platform get compromised?
Pause the problematic source when appropriate instead of moving the same behavior to another domain.