You have an infected machine on the inside of the Firewall that is generating spam and sending it the moment that it's turned on.
If your computers are all clean then the most likely problem is that your IP Address has been Spoofed, if that's the case there is very little that you can do about it as the people responsible can be anywhere in the world and out of your ability to do anything about.
You can also check the Sent Mail Items to see if the supposed mail was actually sent from your IP address as recently I've been seeing several Return Mail Notices to unknown E-Mail address returned to me through one of my ISP's and they are not even showing as originating from me and carry an IP Address that isn't being used.
Col