Pied Piper - sure, that can be the case. But my point still stands - If you have an open relay server running on your machine, you could be the source of spam and virus carrying e-mails without knowing it.
To give an example; if you install Microsoft Exchange Server to run your own mail network, and configure an outbound internet mail feed (which nearly everyone would do), then you have by default created an open relay server. There are many port scanners going round compiling a list of such open relays, and the lists change hands over the internet for certain considerations.
If you have an address of an open relay, it is so easy to configure Outlook express or one of the other mail programs to send mail, which will look at fist glance as if it has come from whatever address you like (could be
[email protected] doesn't have to be valid) but which a trace will reveal came from the machine hosting the open relay server.
There are also several organistaions in the anti-spam business that try and keep a list of open relays, and block mail from them as a service to others.
Cheers