Works email has been playing up and the receptionist has just noticed that every email we recieve gets sent back out !!!

it is going to 'general02/04.10@emailfactories.net' please can someone tell me how to stop this happening as it is getting quite serious, her ebay login details have already been sent there!!, we are using outlook express, i have done a full search with spybot, nothing and have run avg several times and definately have no virus' (as long as avg would pick them up) both are updated to latest definitions. i am just about to install adaware to see if that finds anything , thx in advance.