Reverse Turing Test

You're probably familiar with the Turing test where a computer (AI) is said to have reached a certain level of advancement when it can fool a human being into thinking that it is a human being. It's more complicated than that but you get the general idea.

Well I came across the opposite problem today. How do I convince human beings that I am not a computer? Specifically I'd like to offer my friends a download but I don't want them to think that it's robotic spam message or a phishing attempt.

How do I assure them that it's really me writing the email? So many facts about us are widely available on the Internet. What piece of information or shared experience do I and my friends have in common that is not known? I honestly can't think of anything.

The best alternative I've been able to come up with so far is to let them see that the download is being offered from my own domain and the URL has not been tampered with.