spammers

"They" don't care if the email is delivered or not as it is a server that is throwing out literally hundreds of thousands of emails out at once. These servers do noting more than use harvested email addresses to send spam and do not accept returned or bounced email and if they do they are immidiately deleted. I am sure there are some servers that do verify that an email was bounced from a real address but I am sure those are few and far between. You will also notice that many emails are from fake, non existent, or spoofed email domains so if they do get bounced they aer either going into oblivion or going to a real mail domain as undeliverable. No spammer would send an email from a real domain or they would be promptly shut down.