Relayed spam is when an unknown person uses your email address to send their unsolicited commercial email (spam) to others - not to you. 
This is done by adding a \"worm\" (like a computer virus) to your system or otherwise accessing your email account at your ISP.

You may find out when another ISP emails you to say that some addresses you sent emails to are not available - but you never sent emails to those addresses or when you learn from your ISP that someone has complained about YOU sending spam when you know that you didn't. 
Prevention is advisable but no walls are unpassable - they just lower the risk.

All users of broadband accounts should have a firewall on their system. Two are Zone Alarm annd Sygate.

Everyone should consider installing and using antivirus software (Vet, McAfee or Norton) and anti-spyware programs (Ad-Aware, SpyBot Search and Destroy). These programs must be kept up to date by downloading updates. I check each daily.

If you think your system has been misused, contact your ISP - you want to tell them of a possible breach before someone complains about you \"sending spam\".

Change the passwords on your email accounts at your ISP, in your email program and your modem if it stores a password for you to access your ISP. 
Telstra BigPond advise their customers should do all the above and report the possible problem through the Contact Us form at bigpond.com 
Contact your ISP's Support for specific information about their systems.