I’ve made something like that 15 years ago. It didn’t worked. People receiving the quarantine mails often misunderstood it or dismissed it without reading it.

I’ve made something like that 15 years ago. It didn’t worked. People receiving the quarantine mails often misunderstood it or dismissed it without reading it. We tried with CAPTCHA, without, nothing worked. Our users had to spend a lot of time checking for false positives. In the end we gave up. Oh, and if you want to do something like that, add a reasonable delay for your response as some Spam list check for automatic responses and consider them as SPAM and will ironically block you for that ! Been there...