Open Source Apple Mail Alternatives

K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email. K-9 Mail is designed for both novice and power users and offers lots of features for everyone. It provides features like Push IMAP Support, GPG & PGP/MIME support, Per Account Notifications, Unified Inbox, White and Dark Theme, and Signatures.

Mailspring is a desktop email client designed for macOS, Linux and Windows. It provides modern features like unified inbox, snoozing, templates, offline search, and support for Gmail labels. With Mailspring you can breath fresh air into your mailbox with support for various third-party IMAP provides such as Gmail, Office 365 and Microsoft Outlook. Mailspring intuitive features like read receipts, link tracking, send later, reminders, contact profiles and more.

Mailpile is an effort to reclaim private communication on the Internet. A project to rescue our personal lives from the proprietary cloud and prevent our conversations from being strip-mined for corporate profit and government surveillance. With Mailpile you can take e-mail back and make it yours.

Tutanota is the world's most secure email service, easy to use and private by design. Tutanota brings secure email for everybody.

World's largest secure email service providing end-to-end encrypted email accounts. ProtonMail is based in Geneva, Switzerland and protected by Swiss privacy laws. The software is openly developed with fully open source code.