One of my clients has a need to not allow a certain postoffice to redirect their webmail, as they want to make all of their users use webmail for their primary e-mail. Two reasons for this: they ensure that all of their internal e-mails aren't blocked by each ISP's spam filters, and can update the WebMail UI with banners for important messages to users.
I would like to see the ability to set each of the WebMail restrictions (in Services | Webmail | Properties) for each post office OR perhaps each webmail site (if more than one exists).