Interested to read the comments about virus like worms which hi-jack your address lists to send rogue e-mails. I had an odd experience of this sort last week. It began with an e-mail from something called 'friendgreeting'. Opening it made the screen appear to shudder then spattered coloured laugh words across the screen. The delete key didn't work, but simply closed outlook express and left the message lying in the In box. Thereafter, I started getting failed delivery e-mails about messages I have never sent. I noticed these related to people I have exchanged genuine e-mails with in the past, so my address list was clearly being tampered with.
How do you get out of this kind of fix? With difficulty. I messaged my ISP postmaster (British Telecom) who have not replied to half a dozen messages seeking advice. The Outlook Express help file says that the address list is automatically generated when you reply to any e-mail and cannot be deleted.
My improvised solution seems to have worked, for the time being. I managed to delete the problem e-mails by temporarily moving genuine messages to another file, then using the Select All facility, which enabled the delete key to work. I also found that address lists can be deleted if you access it from the pc start button and accessories, rather than through the Outlook Express toolbar. With all addresses and rogue e-mails now deleted, the problem appears to have gone away.
I should say that, as point of contact for a music society, my e-mail address is accessible through local government websites. Obviously, there will always be some risk in this.
The above might help others if they have a similar experience.