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.