How can you receive spam from an email address that doesn't exist?

When I reply to spam mail and tell them to leave me alone I get a delivery failure notice.

How can you receive spam from an email address that doesn't exist?
Spammers know how to forge mail addresses. You should never reply to spam mails, because that lets them know that they hit a real mail address and subsequently you will even get more spam mail!
Reply:That is just 1 on MANY tricks the spammers have up their sleeves. I could go on and on about their tricks but I have found only 1 perfect system that allows me to stop receiving all spam while not loosing any of the emails that I WANTED to receive. http://www.EnterTo.com has the Challenge Response to protect against spambots AND seamlessly creates an alias channel between you and everyone that you communicate with. They are ridiculously easy to manage as well. I have been using EnterTo.com for about 2 years now and I get NO spam and I get ALL the emails that I wanted to get. There isn't even a bulk or spam folder since it is not needed.

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