I use <kbd>J</kbd> and <kbd>K</kbd> to scroll through emails extensively! Great shortcut, but as is pointed out, there's no way to get past the 50 message limit. When I reach that limit I type this sequence: <kbd>G</kbd> then <kbd>I</kbd> then <kbd>U</kbd> (i.e. 'go to inbox' and 'return to threadlist), then I scroll down if needed to get to the next message I want to read.