[Brian]

Jody, the server-based approach has many advantages to the device-side ones we were using before, but it has a few drawbacks, as well. This is one.

To assemble a link like the mail rule does, the server would need a bunch of information that just isn't as easily available to it. As an example, if you forward a message from your Windows computer, the server would need to somehow know how to assemble a link to that same message on your Mac.

Even if your mac was in your briefcase and hadn't actually fetched that message yet. The mail rule has the advantage of being something that ran inside of Mail, so it had access to that info when it created the action.