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The most pressing question: why does Outlook.com just silently discard some emails?

Not the person you replied to, but as far as I've heard it's done with SmartScreen matches.

This is correct but not the only reason it could be happening.

But why do you consider this good practice? It's (unnecessarily?) frustrating for senders and poses a legal risk for recipients (the sender has the logs to prove that they sent the invoice, while the recipient doesn't have any record).

Again, not the person you replied to. But some feedback mechanisms take time (so action has to be taken after a 2xx reply) and some indicators are just very very very accurate that leaving them in even just Spam is a way bigger risk. Users have a terrible tendency to dig out malware from Spam folders.

This was not the cause in my case (no attachments, no URLs, just plain text, as far as I can remember). I know how to send email (ask mail-tester.com).

Regardless, there are always better options than silently discarding the whole email: delete attachments, erase everything that looks like a URL, even erase the whole message body, but please tell the recipient that you accepted an email and from whom.

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