Gmail - Leave conversation unread when opened by others
You can allow other users to access your Gmail account (Email delegation). It should be noted that when this feature is enabled new messages will be marked as read if they are opened by the other users. How to prevent this from happening?
Sign-in to your Gmail account.
Click on Gear button > Settings.
Go to the "Account and Import" section.
Next to "Grant access to your account:", select "Leave conversation unread when opened by others".