You could work around this by creating yet another hidden namespace with empty prefix. But yea, I guess it's still a bug in Dovecot. Although I'm not
FYI, I tried this. apple's mail works, and thunderbird works, but.. squirrelmail for some reason sees this hidden name space, and starts showing users two sets of heirarchies of the same folders (one as INBOX.*, which was prefered, plus again without a prefix).
exactly sure what separator it should return in that case. Probably either the first non-hidden namespace's, or the inbox=yes namespace's..
Maybe defer this problem to the administrator to define. What seperator to return, when list "" "" is asked for, and there is no namespace with an empty prefix defined. I suspect the number of people this affects is small - folks not using an empty prefix name space, ie idiots like me trying to seamlessly adapt the previous mail server's appearances when moving to dovecot.