One other way is to make the "subject" line specific enough so that it clearly isn't generated by a virus. If I'm sending an e-mail to someone out of the blue (where he or she might think that the message contains a virus), I make the subject line something like "Bob, here's the Smithco data you wanted," on the theory that a virus won't know that "[email protected]" is named "Bob" and won't know that I have a reason to write him about Smithco.