I hear
Spark is the best alternative
Available for both macOS and iOS and it's free
Im skeptical. Never a free lunch so what is their angle? Gmail and Outlook have their own paid email (mostly business) user base in the millions/tens of millions; their apps are to lock you into their ecosystem so having an app makes perfect sense and pays for itself many many times over.
I know Spark has paid teams but they cant be making THAT much off that. So what is their motivation to update the app and run the servers. They claim all types of things they arent doing anything with user data, but I am skeptical.
The other apps are mainly paid (Canary and Airmail). Edison openly says it uses your usage tendencies/statistics to better its AI which is sells the AI to 3rd parties; that is how they make their money. And you can opt out if you wish.
I cannot figure out Spark's angle. Readdle's other apps are paid or subscription; or a mix of those models. They're a 125 employee company, not a charity, so I cannot see maintaining an unprofitable app (even if their other apps are) that has no ads etc for other revenue. I am skeptical they are making enough solely on teams at $5/user/month to counter all of the free users usage.
Or they go the Airmail route and suddenly go al subscription like PDF Expert did.