I see that ~/.local/share/sink/storage/ is messy: filenames and directories with only UUIDs, and mutt -f ~/.local/share/sink/storage says it's not a mailbox. I don't see the telltale tmp, new and cur directories. Isn't the idea to store it that way so that mail clients like kube could interoperate with others?
The problem with most mail clients which know how to download mail is that they all store it in their own way. I think we should start to follow the standard with this project.
~/.local/share/sink/storage/ is not a maildir, it's where we store all sink internal data that are never shared with any other application.
In case of a maildir this will only contain indexes and the actual maildir will live whereever you have it configured.
However, the imap resource will not use maildir as a storage format and will not make the data available to other clients consequently. offlineimap and the maildir resource can be used for that purpose.
Since this is by design I'll close this as wontfix, feel free to ask more questions/add remarks though!