Years ago I was running an email server, (Very low output 3 to 5 users personal email only, no lists) and we had some inbound addresses that were overloaded with spam, so we abandoned them. But rather than just discarding email sent to those addresses ( for fear that someone didn't get the new address) I set them up so (via a piped script in the aliases file ) to fail on receipt with the message "your message to abandoned@email can not be delivered, please use the webform here to send your message"
So we got blacklisted, and checking the logs we had *NO* outgoing email at the time of the accursed spam message(s). The blacklist service didn't give me the whole message, but it contained enough for me to find reference to it in my log.
Near as I can figure, some spammer sent email to us through an open relay, using a honeypot (you get classed as a spammer if you send email to this address ) as his spoofed 'from: address'. My mailer refused to accept the email to the abandoned address, so the relay returned the 'undelivered' message to the honeypot address.
Now I had several problems with this. First, to avoid blacklisting, I had to remove this helpful service. Now those messages go to /dev/null. second, I didn't actually send the email, but we got blacklisted simply because our IP adress was in the chain of Received headers in the email header.
More recently, I had newsletter messages sent to a members of a private club bounced by their local ISP. The sending IP address was not listed in any blacklist I could find. The ISP was just refusing connection, No message, nothing. (I could send email to that ISP from other services like gmail) They wouldn't take my call ( I'm not their customer) so I had some of their customers call and ask "Why am I not getting these newsletter messages?" . I wasn't on the call, but it sounded like they just played dumb. A few of the list members gave us non-local-isp addresses (gmail , yahoo) and now they get the newsletter there.
Again, legitimate email loses out.
And finally, Just about every time, my "password reset" messages end up in people's spam folder. This is one of my most common support calls. (this even after the page where they request the password reset says right on it "check your spam folder" )
There are lots of false positives on spam.