Apple Mail has some decidedly quirky behavior regarding scrolling view focus. One issue that bugs me is that when the message list has focus, the Page Up/Down keys don’t scroll the message list; instead, they scroll the message content pane.
The logic of this design is obvious: the idea is that you can keep the focus on the message list and use the Page Up/Down keys to read long messages. The problem is that this is non-standard behavior. No other app works like this. The standard behavior is that when a list has focus, Page Up/Down scroll the list.
Thanks to the miracle of Twitter, I have been informed of a workaround. I complained, Dave Caldwell (among others) answered. The trick is that if you hold down the Control key, Page Up/Down will scroll the message list.
For what it’s worth, Mail’s help book does have a list of keyboard shortcuts, but this trick isn’t listed. (Nor are Mail’s non-standard focus rules explained.)