[Avamander]

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.

[currysausage]

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.