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The Thunderbird team has released version 155 of its open-source email client, with a better account setup experience, broader OAuth support, and fixes for several mail and calendar issues.
One main improvement is the streamlined manual account configuration. The setup process now handles Exchange, Microsoft Graph, and custom OAuth configurations more cleanly.
Custom OAuth support has expanded to IMAP and SMTP servers. Thunderbird now provides PKCE and external-browser options for custom OAuth configurations. OAuth add-ons can override the application’s built-in settings for selected domains and hostnames.
S/MIME users also get more information in this release. Thunderbird’s S/MIME popup can now display the signature algorithm, digest algorithm, encryption algorithm, and associated key sizes, providing a clearer picture of how an encrypted or digitally signed message is protected.
On the administrative side, Thunderbird 155 introduces new enterprise policies for disabling QR export and experimental features. Administrators can also configure which address book Thunderbird uses by default when contacts are added manually.
Beyond the new functionality, the release includes many fixes. One change addresses IMAP tags that previously failed to synchronize automatically between clients. Thunderbird also fixes an older IMAP tagging problem where changes were not sent to the server and could disappear after refreshing the folder.
Search receives several corrections as well. Attachment filters in global search could previously disappear or produce no results. Body searches could fail with certain quoted-printable messages. Quick Filter has also been fixed to properly consider address book display names shown in message lists and headers.
Thunderbird 155 also improves reliability with encrypted mail. If sending an encrypted message fails, Thunderbird now preserves the draft instead of losing it. Another fix ensures that removing an account clears its corresponding OAuth token from the password manager.
Calendar users get a couple of important fixes too. CalDAV calendars configured for offline use could stop fully synchronizing events, while changes made to cached ICS calendars while offline could be silently lost after reconnecting. Both problems have been addressed in this release.
Several crash bugs have also been resolved, including crashes caused by improper database memory handling, crashes while loading mail folders, and a shutdown crash that could occur while Thunderbird was waiting for OAuth2 password input.
Other fixes cover forwarded messages displaying the wrong encoding, Apple Mail text attachments failing to appear correctly in some MIME messages, Exchange account removal leaving outgoing server settings behind, and various interface and usability problems across Windows and macOS.
For additional details, see the release notes.
