c'mon...
Everyone (should) knows that opening attachments within e-mails that originate from unknown sources are best left unopened. So what could possibly go wrong here? :)
Even so... GPG4Win FTW. That's GnuPG for Windows (uses Kleopatra) and much to my first surprise it can hook directly into Outlook as well. And I'll take GPG over S/MIME any day of the week.