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Author rules to move garbage out of your inbox. Preview shows exactly what each rule catches; Apply sweeps the whole inbox — allow-rules always win, nothing is deleted (moved to Declutter/*, reversible).
New bulk senders you haven’t judged yet. Allow to keep in your inbox, or Junk to sweep from now on. Doing nothing is safe — they just wait.
Older messages in a conversation that already has a newer reply in your inbox — the newest is kept, the rest are redundant. Scans your whole inbox, so Preview may take a moment. Older messages carrying an attachment are left in place (a newer reply may not re-include the file). Nothing is deleted — moved to Declutter/Threads, reversible.