The best open source alternative to Gmail is Proton Mail. If that doesn't suit you, our users have ranked more than 100 alternatives to Gmail and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Gmail are Tuta Mail, Disroot, Posteo and Infomaniak kMail.
Systemli.org is a left network and technics-collective that was founded in 2003. Our aim is to provide safe and trustworthy communication services. The project addresses political activists and people who have a particular need to protect their data.
Fripost – the Free Email Association. Fripost organize the people who want an alternative to "free" e-mail services where you are constantly under surveillance and personal data is handed over to third parties.
Open Source WebMail Lite is a free community edition of full-featured WebMail Pro server application intended for users who need a clean and simple web mail application, which is fast and easy to use. Whether you need a free web browser based email client software for Windows ASP.
Finally, an email service that's built around your privacy. Get your @criptext.com email address and see what it's like to have peace of mind and privacy in every email you send.
Developed to assist small/medium sized business for business emails just like Gmail or Outlook.
Unofficial, free and open-source Google Inbox Desktop App. Built using electron, Inboxer includes a menu bar which is globalmenu compatible and works, and keyboard shortcuts.
Enterprise-level, browser-based suite for IMAP email, calendar, contacts, tasks, notes, wiki, and file management, offering syncing, shared and unlimited resources, multilingual support, customizable dashboard, and integration with mobile and external clients.
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Webmail lite is a front end for email servers. They're not an email provider!