(Not parent author) I believe what makes HEY compelling to some email users is that it is opinionated about what email workflow should be like and fits the bill for many.

While a similar way of using email can be achieved in other mail clients [0] the simplicity of having such features by default it's by itself a good reason.

Some features that are non-trivial to emulate in other email clients are:

- Grouping emails with different subjects etc to the same

- Adding private notes is some emails

- Clipping specific parts

- Some UI specific features (Focus & Reply etc.)

[0] https://klinger.io/post/71640845938/dont-drown-in-email-how-...