SPAMers use phony email return addresses. They make them up as needed, and many times use the domain names of legitimate corporations to help give them an aura of respectability. Lots of times, SPAMers will even hijack the services of someone's poorly configured email server to do their dirty work. Using hacker tools they search the Internet for email servers that have an "open relay" feature, and then exploit it to their own advantage.

With absolutely no connection to a legitimate organization, they make up a phony email address at the legitimate domain, and send out their SPAM messages using someone else's email server. It's easy to forge an email address as coming from anywhere one wants. SPAMers do it all the time, and SPAMstomper™ uses that fact against them.

The phony email addresses a SPAMer uses can't get any email; they're phony. Consequently, when SPAMstomper™ uses the SPAMers return address to send the challenge, it can't be delivered and the SPAMer therefore can't send a confirming email. The original SPAM message gets dropped by SPAMstomper™ and your employee doesn't have to deal with it.
Junk mailers, as opposed to SPAMers, have legitimate return addresses because they are real businesses soliciting via email (searss.org, satellite.tv, joespizzaa.net, etc.), but since they usually use a bogus or bulk email id that no one monitors, or one that can't accept email, they get treated the same as a SPAMer.

I.e. Searss.org sends out a mass emailing for aluminum siding. The sending email id they use is aluminum@searss.org, or some other bogus email account that either can't accept inbound email, or if it does accept inbound email, no one monitors it and any inbound mail ages there till their email system deletes it.

Assume searss.org is on neither the "friendly" nor "nuisance" lists. The inbound email advertising aluminum siding gets trapped by SPAMstomper™. SPAMstomper™ sends a challenge email back to aluminum@searss.org telling Mr. / Mrs. aluminum that their original email won't be delivered unless they reply with a confirming email. Since there is no Mr. / Mrs. aluminum at searss.org, SPAMstomper™'s challenge goes unanswered, and the original offer for aluminum siding is never delivered to your employees mailbox.

If someone is monitoring the aluminum@searss.org account, or if there really is a Mr. aluminum, they have an opportunity to respond to SPAMstomper™s challenge and have the original email delivered. Most mass emailers rely on a numbers game and won't bother to reply to any challenge. Therefore it is unlikely that a junk emailer will even try to reply or that their email system is even set up to accept the initial challenge.

In every day use, SPAMstomper™ eliminates 99% of SPAM and junk email.