Back when I used the Mail app on Leopard and Snow Leopard, it never seemed to tell the Gmail server that to mark as read the messages that I read. This had the annoying effect of requiring me to go to my web browser and mark everything as read there.
Needless to say, I didn't use it much since I would have to go to my web browser to mark it as read anyway.
Does the Mail app in El Captain have this problem? (I switched back to Windows a while back)
EDIT: I want to clarify that I want Mail to automatically mark messages as read as I read them, without having to do anything else. You know, how it normally works with every other email app.
Best Answer
Mail in OS X El Capitan doesn't have this problem.
Mark as Read
&Mark as Unread
affect all of the devices linked to your IMAP account.