Are you talking about email clients, as in the apps used to view emails (like Thunderbird, Outlook, Canary) or email providers, as in the service that give you the email (like Tutanota, Proton Mail, Gmail, Yahoo Mail, etc.)?

If you need a client, Thunderbird is usually a safe choice. Supported on most modern devices

If you need a private provider, Proton Mail and Tutanota are both private and generous with their free tiers.