The app does need to know what your email address at gmail is in order to work.
Other than that there is no need to collect much other information at all.
Google Analytics is currently how I see how users found DarwinMail & where they came from.
Google Analytics also provides info such as users device and browser type.
A list of the data Google Analytics collects can be found here:
https://www.shivarweb.com/2977/what-does-google-analytics-do
Google Analytics is the defacto standard for millions of websites across the web.
The question remains, how else can I see where my users are visiting from, what browsers & mobile devices are they using so that I may better tailor DarwinMail?
I've looked into Fathom before. I quite like what's offered. And the fact that's it's open source is the cherry on top :)
Thank you very much for the suggestion and for giving me a direct link to check out more.
The load times of my site, the quality of my stats and the privacy of my users have all been improved.