In Mail, opening messages with the enter key should result in VoiceOver focusing on the message text and reading it, letting you arrow around or select things as necessary. In El Capitan, though, the old bug where VO instead lands on a scroll area has returned, and gotten worse and worse. This scroll area means you can't arrow through text, you can't interact once to move down to a place in the text and start a say-all from there, you can't select, you can't locate a link or attachment with the arrow keys, and so on. Even if you interact with the scroll area and then with the text inside it, regular arrow keys do not work. You must close and re-open the message multiple times before the text shows up as it should have in the first place. Sometimes--and this is specific to El Cap for me--you will even land on a toolbar, not the message scroll area.