Originally posted by AttaGirl42
It bounces spam mail back to the sender and therefore they think they have an invalid address.
Unless the spammer forged the return address, in which case someone innocent gets flooded with bounces from people running MailWasher as well as bounces from the non-existent addresses on the spammer's list... Although admittedly, the latter would far outnumber the former.

Having watched the flow of spam at work over the last couple of months, and doing a bit of reading through news.admin.net-abuse.email, I can fairly confidently say the following:
- Spammers very rarely use real addresses when sending mail. They try and get people to go to their websites or make phone calls, so their email address is irrelevant. And their lists are generally so dirty that they'd get flooded by bounces.
- Most spammers don't take no for an answer, even when "no" consists of "550 unknown user" from the mail server several times a day, every day, for months or years.

In other words, while fake bouncing might work to get you off a mailing list whose owners are very misguided about their address acquisition but otherwise alright, it won't do a thing against the hardcore spammers.