Wondering if anyone has this issue or knows of a solution. On Yosemite it would group my mail by subject. You could click the newest one and then there was a little arrow in the menu on the right of the message and if you clicked that it would expand and show you the rest of the emails. Since upgrading to El Capitan, this doesn't appear to be working anymore. I have view -> Organize by conversation checked but whether it's checked or unchecked it doesn't seem to make much of a difference. That feature was the main reason I even used the Mail app as opposed to just using a browser and going to my gmail, so I really want to get it back.