Mailwasher - a locally developed spam-filtering piece of software - can be a very useful tool. It can very accurately detect and filter spam out of your email inbox - and we all know how much we dislike spam nowadays.

There is one problem, however. From the aforementioned website one of the 'features' of the software is listed as follows:

• Bounce back unwanted e-mails so it looks as if your email address is not valid. This will make the sender think your address is no longer active so your name can be removed from their list. This unique feature is great for privacy and it couldn’t be simpler!

Heres the rub. THIS WILL GET YOU IN SERIOUS TROUBLE. [click 'read more' to see why.]

The 'Bounce' you see when you send an email to a non-existant address is actually a brand new email, generated by an automated system called a 'daemon' that runs on the mailserver of the domain the message was targetted at. So, the mail processing servers at xtra.co.nz, for example, will have a system for detecting whether 'xyz129@xtra.co.nz' exists, or not. If it exists, and you write a message to that address - it gets delivered. If it does not exist, you get a 'bounce' back - a notification to you that your message could not be delivered as the mailbox doesn't exist.

Simple? Yes. The Internet Service Provider of course, as the people providing that mail server, have the right to generate those bounces. Generally, the return email address on those bounces will be an account managed by the postmaster, or mail administrator, of the domain involved. (Theres still a real person, somewhere, monitoring that address, in most cases.)

Enter Mailwasher. It can 'make the sender think your address is no longer active' through the use of a bounce. So to achieve this, they must be making the bounce message appear as if their ISP is generating it. Uhoh....

Doing this would immediately put you in breach of the Terms and Conditions (T&C) that most ISP hold as the contract you have agreed to adhere to as a client of their services. Why? You're Impersonating their Mail Administrator by pretending to hold their email address, in the sending of your bounce message..

Most ISPs will have a clause which allows them to take action up to and including the complete cancellation of your internet service. This can often seem warranted, too... because... What happens if your bounce, bounces?

It's not uncommon for spam to have a forged FROM address, too. So when your bounce - which is pretending to be 'mailer-daemon@yourisp' itself cannot be delivered, the mailer daemon at their end tries to send a bounce.... except you don't get that bounce. Your ISP mail administrator/postmaster (holding mailer-daemon@ as an address) receives the bounce. Thus your postmaster/mail admin - who has no idea what you're doing - is left trying to understand why they're getting a series of bounced-bounces, where they were not involved in the creation of the original message, in the first place.

It can be a big confusing mess. The key point here is, no matter how appealing it may be to 1) give the spammer a piece of their own medicine by sending their message back to them, and 2) try to dissuade them from sending you further messages by making them think your address is invalid, efforts by anyone other than the mail administrator themselves to do so are quite clearly going to do more harm than good - at your ISPs expense.

So the correct action is? DON'T USE THE BOUNCE FEATURE OF MAILWASHER. DOING SO MAY COST YOU YOUR INTERNET ACCOUNT.

The best thing you can do is use the headers - through tools such as Spamcop or via Mailwasher if appropriate - to make complaints to the appropriate parties - those being the ISPs who host machines used for spam relay. THEY have the power. All the rest of us can do is filter, block, and wait for legislation to catch up.