Ok, so I got around to picking up a free certificate for my email. So now anything I send is digitally proven to have been sent by me (or someone who's had access to my email program. Or someone who know the password I used to log into Thawte and get them to issue me a certificate).
But nobody I know cares about certificates, digitally provable identities, etc. Which means that signing the emails is in fact completely pointless.
Oh well.