You do not need to run Linux to have end-to-end encryption. Apple Mail has supported PGP and S/MIME for years. PGP is available for Windows, as are dozens of clients that support S/MIME. Worried about metadata? Remailers already exist and already provide as much security as is possible (don't trust the remailers we have now? You can run your own, as it only takes one trustworthy remailer to provide security).

"Does this mean Dark Mail is a more or less pointless endeavour"

I still want to know what Dark Mail does that current systems are not already capable of doing. What new design do we need? We have smart cards, PGP, remailers, and Usenet. At worst all we are missing is a nice front-end, and even that is a shaky suggestion. We are faced with an infrastructure deployment problem, not a problem with protocols.