@Hardknox: ISPConfig does not use a folder called Maildir. ISPConfig uses a virtual user setup with its own layout, a folder called Maildir is not needed and not used. Your command will not work with virtual users as all accounts have the same system user.
If he wants to create more folders beside the Inbox automatically, he will have to modify the ispconfig mailbox plugin. There are threads about that topic here in the forum that explain this.