Hi , In most cases the problem will be your email provider that has been hijacked , your email that is online broken in to . who is the provider?
I would like you to download and install a virus checker just to make sure there is nothing virus related .http://www.clamxav.com/index.php?page=dl
you may find PC .exe viruses in your Mail inbox if using Apple Mail or Outlook etc , if so dont worry , they do not affect the Mac at all and are harmless , although sending them to a PC is harmful or if running Virtual PC or Fusion with Windows installed can cause issues with the Mac.
Mostly these virus are found inside Viagra , oil or make money etc junk emails , you wont be forwarding those I doubt to your friends or reading them..
Scan your Mac using ClamX just to be safe .
And make sure your Mac OSX Firewall is turned on for far better security online..
Apple menu , System preferences , Security control panel , Firewall . OSX 10.5 / 10.6
Apple menu , System preferences , Internet & Network , Firewall . OSX 10.2.http://security.getnetwise.org/tools/firewall-osx-instruct
Enable Stealth mode if using OSX leopard in the Advanced options of the Firewall , this will also help with attacks via the internet .
If you leave your Mac online , and the router turned on this helps hackers to break your security , if you turn off your Router when not using it , you will stop many attacks to your router or IP address.
If you have a dynamic IP address over a static one your less likely to have repeated port access attempts on the router or computer if the router is switched off once every night ..
You can also setup MAC address control list or access list also in most routers to improve security , check this is setup if needed please ask , It is a good idea to change the routers password to something other than the default password . , .
Lets see if this helps the issue .
Also is your address book
linked to any other service such as hotmail or yahoo , etc .. your email itself rather than the Mac may have been hacked ? Please give more details , and let me know the results of the scan .
You can track a hacker connection attempts to your Mac in OSX Console or the routers Firewall log connection attempts . Once you know an IP address/s you can use a IP lookup service to look up who the hacker / robot is , or at least where they are in the world to within a Postcode area etc .
You can report connection attempts to your ISP if there is a problem , they may be able to help .http://ip-lookup.net/index.php
This tool can help you see where an IP is , and the ISP provider of connection in most cases . Complaining to the ISP if there is a real issue can help get the access shut down at least .