People should really read the privacy policies before using any email app these days. Many popular ones state that they, at minimum, will keep access tokens on their servers, and many outright cache copies of your messages.
Even if you trust them on privacy or are willing to let them analyze your emails in exchange for some feature, there are very few companies that I trust to handle my email securely.
If a hacker gained access to my Gmail access tokens, they're a password reset link away from taking over most of my other online accounts and probably more than a few banking accounts. And then deleting the password reset emails.
Having worked for large and small companies (including ones that handle emails), security is always second (or third) compared to other business priorities. It takes a large effort to make sure everything is locked down and most companies don't have the skills, resources, and commitment to properly secure everything even if they nominally claim that security is important.
My lack of trust is the sole reason I never had email on my phone until a few months ago. I moved to ProtonMail and I use their app (which is great). I trust them. It's possible I'm wrong, but I know how they make money and if I use them long enough I'll be a paying customer when my free space runs out. They also open-sourced their codebase and I have a friend in law enforcement who has delivered warrants to them and they truly don't keep any data or have access to emails at all.
Most privacy policies are in legalese, and hard experience has shown that what I, as a non-lawyer, think they say is quite often very different from what they actually say.
As a result, I gave up bothering to read them unless they're one or two paragraphs long. I simply assume they say what 90% of them really say: they can take any data they like, do whatever they like with it, and they can unilaterally change the agreement any time they want.
