Mozilla Thunderbird 155 broadens custom OAuth support for IMAP and SMTP, giving organisations more flexibility with modern authentication while improving Exchange setup and administration.
Mozilla’s open-source email client Thunderbird 155 expands support for modern enterprise authentication, with broader custom OAuth capabilities, improved Exchange and Microsoft Graph setup, and new administrative controls.
The release adds custom OAuth support for IMAP and SMTP servers, along with PKCE and external-browser authentication options. OAuth add-ons can also override built-in settings for selected domains and hostnames, giving organisations greater flexibility when connecting Thunderbird to non-standard authentication environments.
Manual account configuration for Exchange, Microsoft Graph and custom OAuth has been streamlined. Exchange account removal has also been fixed to clear associated outgoing-server settings.
Thunderbird 155 provides greater visibility into S/MIME-protected messages by displaying signature, digest and encryption algorithms and associated key sizes. New enterprise policies let administrators disable QR export and experimental features, while organisations can set the default address book used when contacts are added manually.
The release also fixes IMAP tag synchronisation, search and filtering issues, encrypted-mail draft loss, and OAuth token removal from the password manager when accounts are deleted. Calendar fixes address incomplete offline CalDAV synchronisation and lost changes to cached ICS calendars.
Several crashes related to database memory handling, mail-folder loading and OAuth2 password input have also been resolved, alongside fixes for message encoding, Apple Mail attachments and Windows and macOS usability issues.
For organisations, the update strengthens Thunderbird as an open-source alternative for Microsoft-centric email environments while improving compatibility with modern authentication systems.
