Mozilla Thunderbird

Tutanota is an end-to-end encrypted email app with an encrypted calendar & encrypted contacts, all in one place. No one but you can access your emails, calendars, or contacts.

StartMail is an email provider that doesn't collect any user information. Messages are encrypted before delivery to ensure only intended recipients can read them.

ProtonMail is an open-source email provider that respects the privacy and puts your data first, using end-to-end encryption and zero access encryption to secure emails.

SimpleLogin is an open-source email alias solution to protect your email address. You can be anonymous online and protect your inbox against spam and phishing.

AnonAddy is an open-source anonymous email forwarding allowing you to create unlimited aliases and protect your email from spam using disposable addresses.