Mozilla's mail program Thunderbird 159 now also finds the associated calendars when setting up a Microsoft 365 account via the Microsoft Graph interface. Thunderbird has been fetching emails via Graph since version 154, as the team describes in the Thunderbird blog. The program does not migrate existing accounts on its own; you have to set them up again.
What can Thunderbird 159 do with calendars from Microsoft 365?
What's new first of all is the automatic detection when setting up the account, as the release notes state. Mozilla still lists the first calendar phase for Graph as open in its Bugzilla bug database. Completed there are the detection of calendars, the display of events, and their local storage and synchronization. Still outstanding are further synchronization tasks and the handling of time values that Graph does not deliver in world time UTC. Mozilla does not describe how well creating and changing events via Graph already works.
In addition, the version fixes errors with Exchange and Graph accounts. Too many Graph requests previously triggered throttling, causing new messages to appear late. Incomplete information about the outgoing mail server also caused such accounts not to work.
Graph is only available in the regular version, which is released every month. The long-term version ESR 153 only knows the older interface Exchange Web Services (EWS). For Microsoft 365 accounts, switching to the monthly version is therefore necessary, the Thunderbird team writes.
Why are some mail programs constantly asking for the password right now?
Microsoft is shutting down EWS in Exchange Online, that is, for mailboxes in Microsoft 365. Since October 1, 2026, the company has been blocking the interface organization by organization, unless the organization's admins have set otherwise, as the Exchange team explains. On April 1, 2027, EWS ends for good; Microsoft rules out exceptions beyond that.
EWS shutdown at Microsoft 365
Microsoft is blocking the old interface step by step, Thunderbird is switching to Graph.
- October 1, 2026Microsoft begins blocking EWS in organizations without their own setting
- October 27, 2026Thunderbird 159 detects calendars via Microsoft Graph
- April 1, 2027EWS ends for good, re-enabling is no longer possible
Sources: Microsoft Exchange Team Blog, Thunderbird
In a help article from October 21, 2026, Microsoft names as affected, among others, Apple Mail and Apple Calendar on the Mac, classic Outlook for Mac, outdated Outlook versions for Windows, and some backup programs. Such programs then repeatedly ask for the password or report that EWS is not available. Nothing changes for on-premises Exchange servers.
What this means for you
If your Microsoft 365 mailbox in Thunderbird is still running as an EWS account, the move to Graph is worthwhile now. It's best to delete the old account in Thunderbird and set up a new one with the same address, this time with the Microsoft Graph API option. Your emails, events, and contacts in Microsoft 365 remain untouched; only the local copy is deleted.
For events, after the move it's best to create a test event and check in Outlook on the web whether it appears there with the correct time. Until Mozilla gets the time values under control, important client appointments are better placed directly in Outlook. How to organize your calendar and mailbox cleanly there is shown by the Outlook course on TutKit.
With Apple Mail or classic Outlook for Mac, only a program version without EWS helps in the long run; it's best to ask the vendor or switch to a different program. If you manage your Microsoft 365 subscription yourself, you can re-enable EWS until April 2027 via the security setting Baseline Security Mode in the Microsoft 365 Admin Center. The re-enabling buys you time but does not replace the switch.
Mozilla released Thunderbird 159 on October 27, 2026.
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