Rahul put a splash page for Superhuman where the only thing people could do was to sign up for the upcoming product. Once in the system, every potential user got an email with two questions:

1 - What do you use for email today? 2 - What are your pet peeves with it?

The hypothesis was that Gmail got slow and people had to use 3rd party plugins to make it work, while 3rd party clients were buggy and unstable.

Responses from those early users confirmed that both were true, and so Superhuman was onto something.