Re: ..bouncing incorrectly addressed mail
"This is a security issue, as it allows spammers to identify real email addresses in an organization. If it doesn't bounce, it's a real address."
This may have been the case 20 years ago, but not now. Spammers don't care what bounces. They don't care how many million duff email addresses they're using. There's much more work involved in removing the bad addresses from their spam list than in just leaving them in. Sending out the emails either cost minuscule amounts, or zero if you're hijacking someone else's email server. Bounces cost them nothing at all, because they're not coming back to them. So why should they care?
A company has a black-hole for bad addresses simply to avoid the hassle of handling them. It's a valid strategy, but may result in the lose of emails that could have an actual value to your company.