Awesome collection of answers around here! This will mostly be a summary with few additions of what you might be looking for.
EMail provider names that are great to look into based on your needs and threat model: ProtonMail, Tutanota, Mailbox.org, Posteo, Disroot
Mobile apps on GrapheneOS for managing mails: The (Open Source) app of the provider, FairEmail or K-9 Mail
All of those could be perfect for you, but they differ a lot in features, pricing model and design. Also you have to trust that the company remains true to their privacy claims and also won't shut down on short notice like CTemplar did not long ago. And the more people in your environment use a specific mail provider, the easier it gets to have E2E encryption for mails you send to each other. I personally chose Tutanota because I get a well-designed mobile app with a dark mode, E2E encryption, support for custom domains, a lot of aliases and instant notification support even without Sandboxed Google Play Services (which ProtonMail sadly doesn't have). I'm on their old pricing tier though and their current one might be less attractive. They also don't support PGP which I need for one specific use case, which is what I have a free ProtonMail account for.
There's not one correct answer and as long as you stick to some basic rules, any of the mentioned services can be just right for you. Last but not least: Emails will always be inherently less secure than good messengers like Signal, so only use them when absolutely necessary.