I use GPG/PGP to sign my mails, using PGP/Mime, not inline PGP, which creates an ugly mail header that will confuse many readers. My mail has a signature.asc file, and sometimes I get a question about this. Then I explain. The most surprising about this is that most people don't ask anything. Maybe they figure it out, but I doubt that. I do this to introduce the concept to other people.