It had that as well, and undoubtedly it was the main attraction, but iirc it was already not refreshing the whole page when clicking on the email index to read

It had that as well, and undoubtedly it was the main attraction, but iirc it was already not refreshing the whole page when clicking on the email index to read a single email - if not at launch, very shortly after. They later added "html mode" for compatibility with older browsers.