I wonder what the situation would be if you recieved such an e-mail from someone outside the company but did not forward it and deleted it immediately. Would you still be liable for viewing it on a company computer?
Actually the company is liable for not protecting you from this sort of email. If the subject line clearly stated that this was 'vile porn of the highest order' and you opened the message it would not help your case against them though
if you send/forward the mail you breach policy, yes.
if you keep it it could be evidence against the company in your court case