Spam filters: how a mail server decides what lands in the spam folder

Your spam folder catches most junk without ever knowing who wrote it. How does a mail server decide, email by email?

1
Rono's lookalike passed every check, yet one part of the workshop's server almost stopped it. Its job is hard: every day, the server takes in a pile of emails. Most are real, some are spam, a few are fakes like Rono's. Each one goes to your inbox or to your spam folder.
2
And none of the checks can tell which is which. "Please pay this invoice today" is normal from your real supplier and a scam from a stranger. The same words, and the checks pass on both.

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