Open-source email, calendar & chat
MZLA Technologies Corporation (Mozilla Foundation subsidiary) · Communication
Rating
—
No public ratings yet
Trust
6/9
of nine checks verified · Jul 19, 2026
Platforms
2
Windows, macOS, Linux +1
Licence
MPL-2.0
Communication
Independently checked by Altapp where possible; developer-reported data is labeled as such. Last full review: Jul 19, 2026.
Fully open source under MPL-2.0, developed in public by Mozilla's MZLA Technologies.
Mail is stored locally and connects to any standard provider, with encryption built in.
A mature client, but a few verification details remain open.
Verification reflects the version listed above. Practices may change between releases. How we verify ›
Our verdict: a mature, Mozilla-backed open-source email client that keeps your mail in a local store and builds in OpenPGP encryption — now paired with a stable Thunderbird for Android app. The two open caveats are that anonymous telemetry is on by default (with a documented opt-out) and that we found no independent third-party security audit.
Thunderbird is a free, open-source email, calendar and chat client for the desktop that connects to any standard mail provider. It offers a unified inbox across multiple accounts over IMAP, POP, JMAP and Microsoft Exchange (EWS), plus a built-in calendar, tasks and address book, message filters and tags, powerful search, add-ons and themes, and integrated chat over IRC, Matrix and XMPP.
It is developed by MZLA Technologies Corporation, a Mozilla Foundation subsidiary, and released under the Mozilla Public License 2.0 with official builds signed by Mozilla. Your mail is kept in a local store on your own device, OpenPGP end-to-end email encryption is built in, and there is now a companion Thunderbird for Android app (formerly K-9 Mail). Anonymous interaction and technical telemetry is enabled by default in official builds but can be turned off.
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Yes — the pricing listed at our last review is “Free · open source”.
Altapp's independent Trust Report (last full review Jul 19, 2026) records 6/9 of nine checks verified, covering source verification, data & permissions, and known risks. Altapp publishes no score or grade of its own — the count is a tally of what could be independently confirmed, and verification reflects the version listed on this page.
Windows · macOS · Linux · Android.
Proton Mail — each with its own verified Trust Report on Altapp. See the full list ›
