a. Home server. Most people don't have a fixed IP or domain name, so it's going to be a pain for them to access their home server on the run. I do have a fixed IP, but I'd still hesitate to rely on my home connection whilst I'm roaming.
b. Localhost. No one can complain about reliability or accessibility when you are hosting the service right there on your own PC. But now I'm tied to that one PC - I can't check my e-mail from work, or from my phone.
c. Cloud / co-location. Now we have reliable hosting, but privacy?? I'd hesitate to upload my private key to a cloud server. Also, I now need 24/7 internet access even to read my old mail.
Perhaps localhost is the best place for it. My canonical e-mail store can remain IMAP in the cloud, but I can run an instance of Mailpile on each of my devices.
Will the client/server model work on a phone? - Surely most phones refuse to give enough CPU time to apps in "the background". I suppose you could weld a browser instance on to the front of it, and call it a standalone app.
