What about if you receive something like "You have received an e-card from Joe Blow" (I am using that name for any name you might know). In other words, if you know the person who is sending it, or at least if it's from a name of a person you know, wouldn't that be safe?
I had a virus many years ago, it went the rounds in this little corner of Canada, that actually culled names from you e-mail address book, and sent them e-mails from you with the viru . . . so, even e-mail from known recipients is not always safe. That being said, heeding Ross's advice at all times is worth the little extra time and effort to keep yourself clean.