If you try and view an email and have any parameters (e.g. highlighting an attachment), you get the following error message:

—
The following error(s) occurred:

• Unable to determine containing thread for mail with id: 0. This mail does not exist.
  —

In fact, this occurs even if you put a '?' after the mail ID in the URL.

It seems to be related to the hack used for Orion in the MailPathConverter, which puts parameters into the action context when redirecting URLs. Why this only occurs when the '?' is present, I don't know.