Bouncing an email sends a 'non deliverable' bounce message back to the sender of the email so it looks like your email address no longer exists.
Unfortunately bouncing no longer works quite as well with stopping spam as it once did, this is mainly due to the amount of email being sent with forged return email addresses.
Some people report a significant decrease in spam after using Mailwasher to bounce all spam, and sometimes there is no measurable decrease at all.
The problem with trying to bounce all spam messages is that they are either fake addresses, or the address has already been shut down by the ISP.
I suggest using more wildcard expressions in your Blacklist and also use the auto-delete option for blacklisted messages.
The bounce feature is still very useful in some circumstances however, such as when you cannot unsubscribe from a mailing list that you once liked,
or a old contact keeps on sending you large attachments that you don't want for example.