I assume by safe for those handling them his main concerns for DNS via letter would be letter bombs and letters containing toxic substances such as anthrax powder or ricin.
> Asking for return envelopes and addresses seems a little bit terrible :)
How else you going to reply with the DNS Record? ;-)
EDIT: Also thanks for the DNS Via SMS service. I've already used it twice for legit reasons. Who would of thought that having an out of band DNS service would be handy.