I just successfully created 3 subfolders on a public folder using 3 different clients:
- Webtop 5
Of course, you need to be inside the owner group of the folder.
By the way, after creation of the folder, Dovecot doesn't allow to remove it.
I can't find any information inside the logs, but the subfolder has following ACLs:
[root@nethservice ~]# doveadm acl get -u firstname.lastname@example.org Public/redmine/t3
ID Global Rights
email@example.com create expunge insert lookup read write write-deleted write-seen
Maybe @davidep has some suggestion on this behavior, but this not strictly related on what you are looking for.