> The empty space between the rows of email messages seems unnecessarily big. I get this was done for design's sake

Careful when complaining about spacing - when it comes to text, spacing is a functional decision, yes it will make it look pretty when done "right" but this is often also because it is more legible - function and aesthetic are the same when it comes to typography.

Similarly others have complained about the spacing left and right (i.e it being excessively padded to be centered), but this is also for legibility - the very same reason newspapers have columns and not full width text.