Currently, user's home directory is created under a specified folder (e.g. "/_user/admin").
So, if there are many users to create home directory, then it could be problematic in the sense of performance and availability. That is, if there are more than 5000 sub-directories under the /_user/, it will consume much memory and response time.
Therefore, we need a separated interface to play a "home locator" role.
Also, I think the default implementation for the interface should support some predefined variables as most mail servers do. For example, a configuration like '/_user/%2u/%u' can help to locate '/_user/ad/admin' for 'admin' user.