Thunderbird integrates Microsoft Graph API to support Microsoft 365

Thunderbird has introduced native support for the Microsoft Graph API to ensure continued access to Microsoft 365 Exchange Online mailboxes. This update is a response to Microsoft’s planned phased decommissioning of Exchange Web Services (EWS), which is scheduled to begin in October 2026 and reach full discontinuation by 2027.

In the Thunderbird 155 release, the email client also expands its open-source OAuth support for IMAP and SMTP. This includes broader custom OAuth capabilities, PKCE, and external-browser authentication options, providing organizations more flexibility with modern authentication environments.

Additional improvements in the latest version include streamlined manual account configuration for Exchange and Microsoft Graph, new enterprise administrative controls, and enhanced visibility for S/MIME-protected messages. The update also addresses various technical issues related to IMAP synchronization, calendar caching, and database memory handling.