A new option is now available to users when granting access to their Gmail account. This option controls whether a message is marked as read or unread when accessed by the person delegated to access the account. This is useful for the manager/assistant use cases.
How to access what's new:
In Gmail settings, select the ‘Accounts’ tab and navigate to the ‘Grant access to your account section’. When setting up or modifying access, the new options to mark or leave conversations as read when opened by others are available there.
Note: Administrators can control whether their users have this option to grant access or not via the administrator control panel.
For more information: