W32/Sober.O is an email worm. This worm is a variant of W32/Sober. The worm will infect Windows systems and spreads through email.
The infected email carries a spoofed 'From' address picked up randomly from the infected system.
http://www.pspl.com/virus_info/worms/sobero.htm
I'm getting this on all of my email accounts, and a lot of bounces making it look like I'm the one that sent them. You may already be seeing the same.