Hierarchical labels - organizing email

Hello,
We've been using ZMail for a short while now and we find it an excellent tool.
One issue we're having though is organizing emails properly: coming from GMail, we've grown used to using labels in a hierarchical way to sort by client, project, and maybe even subtask. I understand that this functionality is not available in ZMail, right?
So, we've tried using folders hierarchy, which at first sight seemed to cover our needs; but this is what we see:

If you use folders instead of labels, you can't have an email both in "sent" and in the desired folder at the same time. With labels, we can organize everything by client and project, regardless of the email status/folder (sent/archive/inbox).
I've read in the forum that you have in the roadmap a feature to label an email as you compose it. This is very handy, for you can keep the thread organized right from the scratch. The way I see it, a similar feature for folders would be unnatural, you would have the email moved to a folder before actually sending it; and if you do that afterwards... well, it's somewhat cumbersome, and definitely not as speedy as labelling the email as you compose it.
When you receive an email that's part of a chain already organized in a folder structure, you don't get to see the email in context. On the other hand, if it's labelled, you see it right away.
Of course, in some cases you could have a rule to organize emails in folders (though some mails are not that easy to filter), but then you would have your unread/pending mails all scattered through different folders. This may be a matter of tastes, but I feel it's better to have all unread/pending mails in one place (inbox), correctly sorted out by using labels. You could then apply filters/move emails around as you like, if that's what suits you.
We tried mimicking the hierarchy by creating labels such as client/project/task, but the labels' name don't allow special characters. In any case, I think that workaround could be cumbersome as well (imagine having to rename one project).
Also, sometimes we receive one email regarding different projects/tasks, same customer. This is impossible to sort with folders, you need to use labels. So the point is that we can't seem to find a clear separation as to what should be a folder, and what a label, and how to keep labels organized properly without mixing them up (imagine having a similarly named project/task for two different customers).

So, the question is whether you plan to support hierarchical labels, or if you have any suggestion as to how can we organize mails properly and easily. We are open to changing our ways, if someone has a better idea. Right now, we're stalling the migration from GMail for this very reason, since we have lots of emails per client/project and it's really important to us to keep them properly filed.

Thanks a lot for your work, it's a great platform!

Matías.