Sending a *digitally signed* email to prospective employer/supervisor
I cannot imagine anyone being too put off by the digital signature, although I would probably go with a detached signature.

I think if you are going to digitally sign your emails, you may want to make sure you understand what you are doing. This would definitely be the type of thing I might bring up as a "safe" small talk topic during a campus visit. Often the small talk conversations can influence a decision in a tight job search since the search committee is not only looking for someone who can teach the required classes and do great research, but also someone they want to work with.