The best open source alternative to Outlook.com is Proton Mail. If that doesn't suit you, our users have ranked more than 50 alternatives to Outlook.com and 16 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Outlook.com are Tuta Mail, Disroot, Posteo and Infomaniak kMail.
Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any device.
Offers comprehensive encryption for email, calendar, and contacts with zero personal info required. Built on open source, install via Play Store, F-Droid, or APK, with Tor and VPN support. Both free and paid plans; utilizes quantum-resistant encryption ensuring privacy-focused, anonymous usage.
Disroot is a platform providing online services based on principles of freedom, privacy, federation and decentralization.
Web- and IMAP-based email service. 4GB cost 1€/month. Information is hosted on encrypted hard drives. Electricity from renewable resources is used.
Privacy-focused Swiss email system provides 20 GB free storage, supports custom domains and unlimited storage, syncs contacts and calendars, is ad-free, and is part of a secure productivity suite used by over a million users across Europe on all devices.
Wino is a lightweight native mail client for Windows 10/11. It is still in active development. Main motivation for this project is to replace Mail & Calendar for Windows with modern Windows design without using web technologies for speed.
Zero is an open-source AI email solution that gives users the power to self-host their own email app while also integrating external services like Gmail and other email providers. Our goal is to modernize and improve emails through AI agents to truly modernize emails.
Cloudflare Workers-based email solution providing multiple address generation, attachment storage via R2, responsive web interface, RBAC management, Resend bulk email, Telegram or third-party forwarding, verification code recognition, and ECharts analytics.
Forward Email is the best email service, encrypted email service, and fastest email service. It supports unlimited domains and aliases for no extra cost. It uses quantum safe ChaCha20-Poly1305 encryption and offers SMTP, IMAP, POP3, email webhooks, email regular expressions...
Galae is a secure email service focused on deliverability, self-management and interoperability. It totally relies on free/libre opensource software.
0xchat is a secure chat app built on the Nostr protocol. It prioritizes security, featuring private key login, encrypted private chats and contacts, encrypted group chats.
Web-based team collaboration suite combining email client, calendar, contact and task tools, CRM, project and file management, file sharing, trouble tickets, Rocket.Chat chat, and Collabora online editing. Offers open source, self-hosted, and SaaS options.
Switzerland
Germany
EU
Netherlands
Türkiye
United States
China
France
It's not dissimilar to Tutanota. Both these projects are in their infancy, so technically neither is close to a mature product like Outlook, but still worthy of association under the broad category of hosted email-related services