Thunderbird 160, the open source mail client, arrived on 10 November 2026 for Windows, macOS and Linux at once. The interface is untouched. The work sits in four places a mailbox actually lives in: receiving, folders, the address book and the calendar, with more than forty entries in the official list plus one change that only rewrites a sign-in message.

A retention rule that deletes the wrong mail does its work quietly. By the time anyone notices, the message is gone from the server and the local cache may not have kept it either.

The retention rule was clearing the wrong messages

The headline fix concerns IMAP retention settings. Under the old logic the messages that should have been cleared were the older ones, yet newer mail could be removed instead. For anyone archiving through server side rules that is a silent accident: the mail is off the server, the local cache may not hold it, and the loss surfaces long afterwards.

The same update repairs a second uncomfortable case. After a message was deleted the undo button stayed disabled, so there was no way back at all. Notifications were patched twice as well: the tray icon failed to refresh when new mail landed, and certain sender addresses could stall a new mail alert completely.

On the protocol side the fix is in parsing, since POP3 and newsgroup messages that used LF line endings were read wrongly and could shift the body. Sign-in was not repaired but reworded: when a browser blocks cookies and the login fails, the OAuth window now states the reason instead of leaving a white screen that flashes past.

Crashes on a new window, and a message database caught mid-write

Stability brought two hard faults. Opening a new window could bring the program down by itself, the kind of failure a stray click runs into. A crash during a message database update is worse, because a database written halfway is not fixed by a restart. On IMAP two further faults looked like server trouble: a namespace root folder could stay hidden or carry wrong permissions, and on servers with compression enabled folders disappeared after a restart.

Both used to be answered by rebuilding the account from scratch. The corrections now sit on the client side, where they belong.

Folders, drag and drop, and saved searches

Folder handling was repaired in small pieces. Dragging several selected folders moved only the first and left the rest where they were, which makes the gesture pointless. Renaming a folder or moving it elsewhere dropped it out of saved searches, so results thinned with use. Repairing a folder now raises a warning first, especially for directories with offline storage enabled, where the old behaviour started immediately and explained nothing.

Message filters gained a few manners: the dialog accepts Ctrl and A to select every rule, an empty condition no longer matches by mistake, and restoring the default toolbar stopped closing the settings window on its way out. None of these is fatal. All of them grind daily.

Address book and calendar: mostly about formats

The address book repairs almost all concern data formats. The website field used to force an HTTPS prefix onto any non-HTTP link, so an FTP address or an intranet path became a dead link. CardDAV contacts with multi-line street addresses displayed empty and looked lost, the stored format was rejected by some servers and might not read in another client, and photos kept in vCard 3.0 sometimes refused to show.

The calendar has its own list. A delete button in a read-only calendar stayed lit and did nothing when pressed, CalDAV reminders could be neither switched off nor postponed, and some webcal subscriptions were blocked as unsafe sources. Attendance options for recurring events were missing altogether, the task list stopped updating after the toolbar was customised, and an invalid ICS file was overwritten when an event was edited.

Who should take it

Anyone running Thunderbird on IMAP with server side retention should move now, because deleting the wrong mail costs too much. People who keep several accounts in one client and read unread counts from the badge will feel the difference at once, since several accounts used to be undercounted. Heavy calendar users have their own reasons, as reminders that cannot be closed and subscriptions blocked as unsafe are daily friction.

On POP3, with a plain folder tree and no calendar, little of this will be visible, and waiting for the next release is reasonable. Thunderbird is maintained by MZLA under the Mozilla Foundation, released free and open source, and puts mail, calendar, address book and chat in one window with the same features on all three desktop platforms.