My email opens at reduced size, but the expansion icon is grayed out, so I cannot get the page to full-screen size.
There’s one of two causes, the more likely of them being that you’re already in full-screen mode with Mail. When you’re using full-screen mode for the Mail app, double-clicking a message slides it up from the bottom, and only the red close and yellow minimize buttons remain available. The green full-screen/zoom button is grayed out, as you described.
In normal use in El Capitan when hovering over the green button, it should show full-screen expansion arrows (pointing diagonally to the upper right and lower left); before El Capitan, hovering over would reveal a plus sign, which, when clicked, would maximize the window to the current display without going full screen. (Hold down Option and you can access the old behavior in El Capitan when clicking.)
If that’s not the case, the other possibility is that you’ve set the interface to only show gray control buttons. In the General system preference pane, selecting Graphite instead of Blue from the Appearance pop-up menu changes all the control buttons to shades of gray, but hover-over insets, like the x in the close button, still appear.
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