In the past this was a simple situation where a spammer picked up your e-mail address, and bombarded it with annoying sales offers. The Spam was sent from a fictitious e-mail address, so you were unable to trace where it came from, or respond to tell them to stop (not a good idea anyhow, as that confirms they have a valid recipient).
One way to filter this unwanted mail is to detect whether the sender address is a valid address, or simply check if it is from a person you know, and dump all others.
To get around this the spammer has changed tactic, instead of harvesting your e-mail address to send to you, they now harvest your valid e-mail address to use as their sender address. The spammer then sends their lurid sales offers to hundreds of thousands of annoyed people, as if they were messages from you. You do not see this message, your only involvement is to receive the "bounce" messages or responses from annoyed receivers of this Spam.
Neither you, your ISP or your e-mail provider can prevent this e-mail being sent, as it is not actually passing through any mail servers that you use, it is being sent from somewhere the other side of the world, to somewhere else on the other side of the world, just using your e-mail address as the sender's address. The only step where your ISP or e-mail provider can help is to try to stop the bounce messages annoying you.
We have received a number of messages from users of the @nzart.org.nz e-mail service, who are experiencing his problem. NZART does not provide an e-mail server at all, we just provide an alias forwarding service. Users of the service are not sending e-mail through the NZART e-mail server, they send through their own ISP. The service simply readdresses the reply you get back to your correct ISP address. The spammer that is using your e-mail address as a false sender address is not sending their Spam through the NZART e-mail server, they are sending to their (or some unsuspecting unprotected ISP elsewhere) server, only the bounce message that is being sent back to you is coming through the NZART server redirection.
The only change we can make on the NZART server that could help the situation is to filter out all bounce messages. While this would reduce the impact on our users, it would not stop the Spam going out in your name (as it happens on the other side of the world) it would only stop you knowing about it. The undesirable result would be that all bounce messages would be stopped so if you sent an e-mail to a friend who has changed e-mail address you would never receive a bounce message back, and would not know it had failed to be received.
Some users who have received many many of these spammer bounce messages have asked us to remove their address from the NZART server. We always comply with this, but while it will stop the user seeing the messages, but will not stop the actual Spam going out, so your name continues being used.
The spammers get these e-mail addresses from many sources. Most are from automated browsing around the Internet picking up anything that looks like a valid address within the text of a web page or document on the Internet. Other sources are MOST -"free"- web services that require your e-mail address to register -entering a valid address into these is almost certain to be adding your e-mail address to a spammer's list. Another source is virus software that will read an address book on your (or a friend's) infected computer, and send all the addresses you have stored, to the spammer for their list. Other smart spammers simply recognise the pattern of amateur radio callsigns, and try all valid combinations. One thing we are certain of is that the NZART server is not a source of e-mail addresses for spammers, it is not possible (even for the administrators) to extract a list of all @nzart.org.nz e-mail addresses from our server. Spammers share their lists, buy them and sell them, so once your address is on a list, it will be used by many.
In summary, Spam is the scourge of the Internet. The recent trend to use unsuspecting people's e-mail address as the sender address, is very annoying, but once it happens, you cannot stop it, your ISP cannot stop it, your e-mail provider cannot stop it. Changing your e-mail address does not stop it, you just don't notice it happening any more.
The future may include a new method of sending e-mails that includes a validation check back to the sender server every time a message is received, which confirms it came from where it said it came from. Unfortunately this is not even close on the horizon yet, as it requires a level of co-operation between software developers, ISPs and network companies that does not exist yet. We all look forward to the day something like this is implemented.