When you say, “they added their own app” are you referring to a program that is installed on a local PC? That’s how I view, file and compose my emails, whether it be Thunderbird or Outlook.
"When you say, “they added their own app” are you referring to a program that is installed on a local PC?"
Yes. It has it's own program, and it is simple, clean, and slick. They have it for Windows, Linux, Mac, iPhone, iPad, and Android.
It also include their calendar (hopefully, soon they will include 'Drive' too, you can use on your browser), which is a storage space (encrypted) that you can use. I store my important and backup files there).
Here is what the app looks like (in dark mode):
(Plus all of the email addresses that I imported are below the folder (about 15, or so).)