More and more email filter applications are hopping onto the bandwagon and offering the ability to 'bounce' email messages by replying to a chosen sender with a crafted bounce message. These bounce messages are emails made to resemble error messages generated automatically by an email server when a sender tries to send an email to a non-existent recipient.

The aim of bouncing is to alert a sender to the fact that their intended recipient's email address does not exist. The main intended use for such a facility is to bounce spam emails in the hope that the spam recipient's email address will be removed from the spammer's mailing list. Of course, you could also bounce an email that you've received from someone whom you wish to ignore and make it appear as though your email address doesn't exist!

For this manual bouncing technique to work, an assumption has to be made that the recipient of a bounce message will act upon it. If the recipient of a bounce message decides to do nothing or is unable to do anything then the bounce is wasted. In the case of bouncing spam messages this is likely to be the case. Let's look

1. Spammers often send out hundreds, thousands or even millions of emails at a time using anything from Microsoft Outlook Express to Open Relays (other people's publicly accessible email servers) to third party specialist 'marketing' companies. The likelihood of the spammer deleting a small proportion of non-existent email addresses from his list is small especially when they are well aware that the bounce email could have been manually crafted. There's very little that the spammer could gain from deleting a non-existent address.
2. Spam emails often have parts of their header information forged. This includes the reply-to address header to which any bounce messages will be sent. If the reply-to address exists then it's likely to belong to some innocent bystander who is going to be perplexed by your bounce message (if they never sent the spam to you in the first instance there's no reason for them to expect any replies from it). Consider how the innocent recipient might feel if more than a handful of the other spam recipients should also decide to bounce the same email. Potentially, each person bouncing could be contributing to a distributed denial of service attack against the innocent victim!
3. In the rare event of the spam email's reply-to address actually belonging to the spammer, if he can identify your bounce message as a fake then you will have done exactly the opposite of what you intended and confirmed your email address to be live.

In light of this, there doesn't seem to be much to gain from bouncing spam emails.