Stock email app with Gmail can't mark "as read" automatically

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Hi,

First of all, sorry for my bad english.

I use the LG email app to sync my Gmail inbox. I have another IMAP account to sync and I don't want to use two different apps for my emails, that's why I don't use the Gmail app.
So I configured my Gmail account with Exchange. It works well, I can receive all my emails instantly. But when I read an email, it won't be shown as "read" in my desktop email client or in the Gmail webclient (but it's marked as read on my phone, of course). I have to "refresh" manually my emails on my 4X HD to get those email as read on the Gmail server.

Is there a settings I have to change to solve that ?

Tell me if this is not understandable enough.

Thank you