Ah, I’ve just re-read your message thoroughly.
I looks as tho…
1. Under Yosemite, mailboxes need to be created one at a time. Trying to create an already existing mailbox throws an error.
e.g. ‘Year 2016’, then ’11’ in ‘Year 2016’, then ’17’ in ‘Year 2016/11’, and needs a slash, but would probably work if the Engineers suggestion of ‘at beginning of mailbox “Year 2016” was used.
2. Under El Capitan, trailing slashes are required, and an entire hierarchy of mailboxes can be created in one go, failing silently if already existing.
e.g. make new mailbox with properties {name:"Year 2017/12/17/"}
Thanks
Regards
Santa