I would simply like to manually mark an email as "opened" in the list of "unopened emails" when I learn that the recipient has read it—for example, if they tell me over the phone or reply via email.

In my opinion, this would be a useful option for all users.

This could be implemented by opening a menu with a right-click on the email entry. Currently, only the "undo" option appears, but an additional option like could be added here.