I cant believe I'm saying this but Outlook.

I used to not like Outlook at work, but when I worked for a startup they used GSuites and the Google mail experience is so bad I honestly couldn't believe that anyone would be using it. Threads are mixed together when multiple people reply its impossible to read content anymore and they do also change the formatting of content which puzzled me the most.

Maybe for people that only ever used Gmail it's okay, because they are used to its weirdness and have never seen a more structured and visually appealing way to read email.