Wonder why I never thought of something like this. A few weeks ago I forgot to email a presentation to a professor, so I took a screen-cap of my sent emails, messed around with it in paint, printed it, and showed her. I told her you can't trust technology these days xD.
I'm sure this would work if you made it look all official, like check the option where it shows the site URL and time printed. I might try something like this this weekend.