Should probably mention that next you your statement about privacy. I would consider an email service that has Google analytics on it not terribly private
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.