Thunderbird e-mail client does collect some basic information about installation numbers, versions in use, crashes, add-ons, etc that relate to technical data and which does not identify individual users. This defaults to ON, but can be disabled by users.
It does also have a category called "highly sensitive or clearly identifiable information" that defaults to OFF. This requires users' consent with advanced user notice and an opt-out.
So as things stand, users have pretty good control over their Thunderbird e-mail clients for privacy.
Betterbird may of interest too as it is a soft fork of Thunderbird ESR version, and retains compatibility with the same version of Thunderbird (you can switch between them), where Betterbird has fixed some Thunderbird glitches and bugs (and which they pass back to the Thunderbird project), and they default all telemetry to OFF.
Betterbird is headed by Thunderbird's former maintainer who has been joined by a group of people who share the view that users deserve a better experience and who do not agree with the approach of the Thunderbird management.
Chat accounts for IRC, Matrix, XMPP and Odnoklassniki can also be managed from within the client, and even RSS news feeds.
See
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Betterbird. Simply better. A soft fork of Mozilla Thunderbird
