If you’re using Apple Mail after upgrading to Yosemite, the initial setup will require an upgrade of your old email mailboxes. The Mail Upgrade box is shown below.
If you click on the Read more… link to learn about new features in mail, it’s possible you’ll get a message stating, “The selected topic is currently unavailable. The topic you were trying to view could not be found.” This is shown below.
Close this message, and after waiting a little while, try clicking the Read more… link again.
You’ll notice that the Mail Help window does not have the same padding (space) above the window control buttons (red, yellow, green dots). Notice that the top corners are squared off rather than rounded. See below for a visual of this. This is likely an oversight by the programmers.