Thunderbird 160 Fixes Wrong IMAP Message Deletion and Microsoft 365 Setup Issues

Mozilla's official release notes list Thunderbird Desktop 160.0 as released on November 10, 2026. The list covers mail, account setup, address books, filters, calendars, the interface and security. In Linuxiac's coverage, Bobby Borisov described it as fixing IMAP message deletion, disappearing folders, Exchange account configuration, and calendar problems. The official notes support that description.

The retention bug that deleted the wrong mail​

Thunderbird's retention settings exist to remove older mail and keep recent messages. In Linuxiac's summary, the bug could have caused Thunderbird to delete newer messages rather than older ones. Mozilla's notes describe the same problem in one line.

There are two important limits:

• Mozilla says nothing about data recovery. The notes do not say that version 160 restores messages the bug already deleted. Don't assume it does. If you think you were affected, check your server's deleted-items or retention options and your backups.
• Mozilla doesn't say who was affected. The notes name no server products, settings or frequency. You should treat this as a confirmed fix, not as proof that every IMAP user lost mail.

Practical step: If you use retention policies on IMAP folders, update before the next cleanup runs. Then look at your retention settings again.

Folders that disappeared after a restart​

The second major IMAP fix involves folders that could disappear after Thunderbird restarted, when the server used compression. Betterbird, a downstream fork of Thunderbird, has published an analysis of the Thunderbird 160 release. It lists this bug as a regression: "Thunderbird 159.0: IMAP folders disappear after restart despite successful LIST responses, IMAP COMPRESS=DEFLATE". Betterbird traces the cause to a change made in Thunderbird 159.

In other words, Thunderbird 159 broke this and Thunderbird 160 fixes it. If your folder pane emptied out after the last update, this is the likely cause.

Betterbird has an obvious interest in criticizing upstream release quality, and its own figures are inconsistent. It reports a total of 58 bugs were fixed, 14 of them regressions in one place, and a smaller "6 of 58" in its summary. Even so, its point is reasonable: a noticeable share of each monthly release goes to fixing problems the previous release caused. Betterbird also attributes the Undo fix to a regression introduced during the 160 development cycle.

Other mail-handling fixes in the release:

• Undo after deleting messages: Undo stayed greyed out, so deleted messages couldn't be restored that way.
• Saved searches: Renaming or moving a folder could remove it from saved searches.
• Multi-folder drag: Dragging several selected folders moved only one of them.
• IMAP namespace roots: These folders could be missing or have the wrong permissions.
• Crashes: Opening a window, or updating a message database, could crash Thunderbird.
• POP3 and news parsing: Messages with LF line endings could be parsed incorrectly.
• Notification freezes: Some sender addresses could freeze new-mail notifications.
• Reply-To: Replies to saved messages could be blocked even when the Reply-To address was valid.

There is also a Windows-specific fix: the notes list a case where the Windows system tray icon failed to update when new mail arrived. That one matters if you rely on the tray icon to know when mail arrives.

Section summary: Version 160 repairs a retention bug that could delete mail, fixes disappearing folders on servers using compression, and corrects several problems with Undo, folder management and crashes.

Exchange and Microsoft 365: what changed​

This is the most relevant section for Windows administrators. Thunderbird 160 changes several things about how it connects to Microsoft accounts:

• Exchange autodiscovery no longer fails when the server redirects to a different email address.
• Microsoft Graph account setup no longer fails for tenants without a SharePoint license.
• Office 365 account setup no longer automatically offers EWS configuration.
• OAuth sign-in windows now explain when blocked cookies may be preventing sign-in.

Mozilla's beta notes show the EWS change and the OAuth wording change were already in Thunderbird 160 beta 5, which released November 4, 2026.

The EWS change makes sense given Mozilla's own September 2 announcement. In that post, the Thunderbird team said Microsoft had announced a phased shutdown of EWS on Microsoft 365 and Exchange Online. The shutdown starts in October 2026, and Microsoft aims to finish it in 2027. Offering EWS during Microsoft 365 setup now would steer new users toward a connection method that is being switched off.

The same post gives Thunderbird's migration guidance:

Mozilla also said Thunderbird can't switch an existing account from one protocol to another. To move a Microsoft 365 account from EWS to Graph:

1. Delete the EWS account in Thunderbird's account settings. Mozilla says this only removes the local account and doesn't affect data stored in Microsoft 365.
2. Open Account Hub and enter your Microsoft 365 email address.
3. Choose the Microsoft Graph option when Thunderbird detects that the account is hosted on Microsoft 365.
4. If automatic setup fails, follow Mozilla's manual configuration instructions. Organizations using custom client and tenant IDs will find the required API permissions on Mozilla's wiki page for manual setup.

Two warnings from that post still apply:

• ESR users don't have Graph. Graph support first shipped in Release-channel version 154. The Extended Support Release, version 153, supports EWS but not Graph. Mozilla said Microsoft 365 users on ESR need to move to the Release channel to keep using their mailboxes.
• Graph covers email only. Mozilla said Thunderbird didn't yet support Exchange or Microsoft 365 calendars or address books through EWS or Graph. The 160 release notes don't add that support, so the CalDAV fixes below don't change this.

Section summary: For Microsoft 365 users, Thunderbird 160 removes friction from Graph setup and stops offering EWS during Office 365 setup, as Microsoft shuts EWS down. If you connect over IMAP and it works, you don't need to change anything.

Contacts and filters​

There are three fixes for CardDAV and vCard contacts:

• Contacts were sometimes saved in a format that some servers didn't support.
• Some contacts with multiline street addresses appeared empty.
• Photos saved in vCard 3.0 contacts could fail to appear in other apps.

Filters get two fixes:

• Empty filter conditions could cause incorrect matches.
• Address book conditions failed to match messages with missing addresses.

The only feature the release notes list as new is a keyboard shortcut: Ctrl+A now selects all filters in the Message Filters dialog. It's small, but anyone who has managed dozens of filters one at a time will appreciate it.

Calendar fixes​

The calendar fixes include:

• CalDAV reminders that couldn't be dismissed or snoozed.
• Missing attendance options for some recurring event series.
• Some webcal subscriptions that were wrongly blocked.
• Invalid ICS calendar files being overwritten when events were changed.
• Delete Event staying enabled for read-only calendars in the Today Pane.
• The task list no longer updating after toolbar customization.

Betterbird's analysis traces the webcal blocking to a much older problem. It lists a bug in which a webcal calendar can't be added - console says blocked by OpaqueResponseBlocking, and dates the cause to Thunderbird 108.

Security and smaller fixes​

Mozilla lists "Security fixes" without naming CVEs, severity ratings or affected components. That's a reason to update promptly, not a reason to assume any particular threat. The release also includes visual and UX improvements, News Reader and Feed Reader fixes, and a fix for macOS Dock badges that left out unread counts when several accounts were set up.

Should you update now?​

Yes, especially if any of these apply to you:

• You use IMAP retention policies.
• Your IMAP folders disappeared after Thunderbird 159.
• You're setting up Microsoft 365 through Graph, especially in a tenant without SharePoint licenses.
• You depend on CardDAV contacts, CalDAV reminders or the Windows tray icon.

Thunderbird 160 is available for Windows, macOS and Linux. Release-channel users get it through the normal update process. ESR users won't get these fixes the same way, and if they use Microsoft 365, the ESR channel also lacks Graph support.

Mozilla's notes don't promise that version 160 fixes every Exchange or OAuth sign-in failure, and they don't promise to bring back mail the retention bug deleted. They are a list of behavior that has been corrected. Update first, then check your retention settings and your mailbox.