> Just downloading isn't enough, would require sync back (and that's
> gonna be difficult). And having to download several Gig is not
> an option for me.

I don't know what kind of storages are you talking about, but the way
is done by the mail system is just a read-fs and a smtp client, you do
not write into /mail/fs/mbox to send mail :-?,

> It has to be an synthetic filesystem, since the server should do
> most things. As already said: clients should only speak mailfs-ng,
> and not doing their own storage handling anymore. This also includes
> things lke filtering, searching, etc.

why do not start with what we have now? making upasfs to use the
remote storage, making it able to order mailbox and move messages
between them and those things? that's why i suggested taking a look at
what Russ said about upasfs from p9p :-? starting from scrach would be
much more painfull :-?