is this through an exchange server?
i think one of the virus scanning softwares for exchange server marks files like that when it cleans a virus from an email
check the
virus encyclopedia at trend micro
our exchange server uses their product, i think i have seen messages similar to that, maybe
yeah, interscan is that name of their email server virus
scanner, they have it for NT or unix, so its likely that your email provider has their software running and it stripped virus attachments from those four emails