Spam Hound checks that people sending you email actually exist.

When you receive an email from an address that you have never received an email from before Spam Hound automatically checks that address to see if it is valid.

Spammers almost never send email from their real address.
Usually the return address is completely made up.
Spam Hound detects valid and invalid email addresses and factors this in when deciding if a particular email is spam or not.

Some spam filters offer to "bounce" spam for you.
This is a bad idea for a number of reasons:

1. It assumes that the return address of the email is a valid address.
   This is almost always not the case.
2. It assumes that once the spammer receives a bounce they will take you off their list.
   This is also unlikely since spammers know about this trick and in any case do not find it worth their while to keep their lists updated.
3. Often the return address is does not belong to the person who sent the email. Often a spammer will choose the email address of an innocent third party to use as the return address. As a result that person gets buried in a mountain of bounced email. By faking a bounce you are only adding to the problem.
4. The bounce address can be different from the return address of an email.
   By bouncing to the return address you can confirm to the spammer that your address is valid.