An open source mail client lets you read the code that holds your password, and it can never lock your mail behind a subscription. There are more good ones than people think. Here are six worth installing, ranked. We made the first one, CitrusMail, so check what we say against the others' own pages.
Open source email clients at a glance
| Licence | Mac | Windows | Linux | Calendar and contacts | Read receipts | |
|---|---|---|---|---|---|---|
| CitrusMail | MIT | Yes | Yes | Yes | Yes, CalDAV and CardDAV | Yes, free |
| Thunderbird | Open source | Yes | Yes | Yes | Yes, plus tasks | Not on its feature page |
| Betterbird | Open source | Yes, English | Yes | Yes | From Thunderbird | Not on its home page |
| Mailspring | GPL-3.0 | Yes | Yes | Yes | Invitations now, calendar later | Pro, $8 a month |
| Evolution | Open source | No | No | Yes | Yes | Not listed |
| KMail | Open source | No | No | Yes | Via KOrganizer and KAddressBook | Not listed |
1. CitrusMail: mail, calendar and contacts in one modern window
Our pick, and our app. CitrusMail is a free desktop app for Mac, Windows and Linux that opens the mailbox you already have, from Faivelo or any IMAP or POP3 server. Your calendar (CalDAV) and contacts (CardDAV) sit in the same window, so the week and the people are one click from the inbox. You sign in with an email and a password; it finds the server settings for you.
Three things set it apart. Read receipts are built in and free: track a message and Sent shows Delivered, Opened or Bounced on it. You can bring your own AI key from Anthropic, OpenAI, Google, xAI or Perplexity for summaries, AI Reply, questions across your mail and translation in both directions. And it looks like an app made this decade, with a signature for each mailbox you send from.
The code is MIT, so you can also put your own name and colours on it and hand it to your team as your own app. The trade: it is new. Version 0.1.0 has no phone app, no unified inbox across mailboxes and no PGP, and it is built on Electron. Download CitrusMail and see if it fits.
Pros
- Mail, calendar and contacts in one window
- Free read receipts: Delivered, Opened, Bounced
- AI and translation with your own key
- MIT, and you can brand it as your own app
Cons
- Version 0.1.0, so young
- No phone app or unified inbox yet
- No PGP
2. Thunderbird: the mature all-rounder
The one most people know, and for good reason. Thunderbird is free, open source and funded by donations, with email, calendar, contacts and tasks for Windows, Linux and macOS, plus an Android app. It has a unified inbox, a large add-on catalogue and more than 20 years of work behind it, by its own account. If you need an extension for something unusual, it probably exists.
Where it shows its age is the feel of it. It is deeply configurable, and that means more settings to find. Its feature page does not list read receipts or AI help. Pick it if you want the safest, most extensible choice and do not mind tuning it.
3. Betterbird: Thunderbird with extra fixes
Betterbird calls itself a fine-tuned version of Thunderbird. It adds features and bug fixes of its own and can run on the same profile as Thunderbird, so trying it costs nothing. It ships a Windows installer and a portable build, a Linux archive and a Mac disk image in English, and its latest release came out on 27 October 2026. Choose it if you like Thunderbird but want its annoyances fixed sooner.
4. Mailspring: polished, with paid extras
Mailspring is a good-looking client for Mac, Windows and Linux, licensed GPL-3.0, with a unified inbox, fast search, signatures, translation and undo send. The free version covers everyday mail. Read receipts, link tracking, send later and templates are in Mailspring Pro at $8 a month. It shows calendar invitations and lets you RSVP; its site says a full calendar is coming later this year. Pick it for a clean inbox across many accounts.
5 and 6. Evolution and KMail: strong picks on Linux
On a Linux desktop, two clients deserve a look. Evolution, from GNOME, brings mail, calendar and address book together and can reach Microsoft Exchange through its EWS extension; its 3.62.1 release is recent. KMail, from KDE, supports POP3, IMAP and Exchange (EWS), has strong OpenPGP and S/MIME support, and works with KOrganizer for meeting invitations. Its newer versions can translate offline. Neither offers a Mac or Windows build on the pages we read.
Geary, GNOME's conversation-style client, is pleasant too, but its last release on Flathub is from more than two years ago, so we left it off the ranking.
How to choose
- You want one modern window. Mail, calendar, contacts, read receipts and AI without add-ons: CitrusMail.
- You want every option. Add-ons, tasks and two decades of settings: Thunderbird, or Betterbird for its fixes.
- You live on Linux. Evolution for GNOME and Exchange, KMail for KDE and encryption.
FAQs
What is the best open source email client?
For one modern window with mail, calendar, contacts, read receipts and AI, we think CitrusMail. For the most add-ons and settings, Thunderbird. On Linux, Evolution and KMail are both strong.
Is CitrusMail really open source?
Yes. The code is MIT licensed and on GitHub. You can read it, change it and ship your own copy under your own name.
Does CitrusMail work with Gmail or Outlook?
It works with any IMAP or POP3 mailbox that sends over SMTP, and it knows the settings for Gmail, iCloud, Outlook, Yahoo, Fastmail, Zoho and others. Some providers ask for an app password; CitrusMail tells you when.
Which open source email client has read receipts?
CitrusMail has them built in for free. Mailspring offers them in its paid Pro plan.
Try the newest one first
CitrusMail is free and takes a minute to set up: download it, type your email and password, and your mail, calendar and contacts are there.
- Mac, Windows and Linux
- Free read receipts
- Bring your own AI key