My wife's e-mail (Outlook Express on a Win-XP system) is receiving hundreds of "Mail Delivery Failure" messages every day for e-mail she never sent to names that are not in her address book and that she otherwise does not know. There is nothing related to any of these returned e-mails in her 'Sent' folder. Most of these e-mails appear to have a .dat file of a couple of hundred bytes or so attached. Her e-mail address appears to have been spoofed into the 'Return-Path'.
Does this indicate a virus in her system somewhere receiving e-mail addresses from some bot somewhere, sending them from her computer, and somehow avoiding copies to the 'Sent' folder? Aside from manually deleting the returned e-mails, is there anything that can be done to keep them cleaned out automatically (a filter of some sort?) and to fix whatever is causing this to happen?