Fastmail and Outlook have no built-in calendar sync, because Outlook does not support CalDAV, the protocol Fastmail calendars use. Outlook can subscribe to a read-only link to a Fastmail calendar, which refreshes on Microsoft's schedule. Keeper.sh, a calendar sync service, copies Fastmail events onto an Outlook or Microsoft 365 calendar as busy blocks and keeps the copies current.
Why Outlook cannot sign in to Fastmail calendars
Outlook has no CalDAV client, so it cannot add a Fastmail calendar account the way an iPhone or Thunderbird can. Fastmail's own Outlook setup page says Outlook does not support CardDAV or CalDAV and that calendars need a third-party tool to sync (Fastmail: Set up Outlook, checked 29 October 2026).
Fastmail's calendar server is https://caldav.fastmail.com/, and it accepts only an app password, never your regular Fastmail password (Fastmail: Server names and ports). Keeper.sh signs in there with an app password.
A read-only link also works in reverse, from Outlook into Fastmail. What is calendar sync explains how a subscription differs from a copy, and the Google Calendar sync guide covers other provider pairs.
Subscribe to a Fastmail calendar in Outlook
A subscription shows a Fastmail calendar inside Outlook as a separate, read-only calendar that Outlook refreshes on Microsoft's own schedule, which how to sync Google Calendar with Outlook gives with Microsoft's figures.
Publish the calendar in Fastmail
In Fastmail on the web, open Settings > Calendars and edit the calendar. In the Publish section, turn on Full event details or Free/busy information, copy the link Fastmail shows, and click Save (Fastmail: Calendar sharing via public URL, checked 29 October 2026). Free/busy shows only when you are busy, with no event details. Anyone holding the link can view the calendar, and Reset URL replaces a link that has leaked.
Add the link in Outlook
In Outlook.com or Outlook on the web, open Calendar, choose Add calendar, then Subscribe from web, paste the Fastmail link and select Import (Microsoft: Import or subscribe to a calendar).
New Outlook for Windows uses the same Add Calendar > Subscribe from web steps. Classic Outlook uses Open Calendar > From Internet. If either desktop app returns to the calendar without adding anything and shows no error, Microsoft's workaround is to add the link in Outlook on the web instead, and the calendar then appears in the desktop app under Other Calendars (Microsoft Learn: Can't add an Internet calendar).
Show an Outlook calendar in Fastmail
The reverse route is also a read-only subscription. In Outlook on the web, open Settings > Calendar > Shared calendars, choose a calendar and a level of detail under Publish a calendar, select Publish and copy the ICS link (Microsoft: Share your calendar in Outlook on the web). In Fastmail, open Settings > Calendars, paste the link under Subscriptions and click Subscribe to calendar (Fastmail: Public calendars). Anyone with a published link can view the calendar, and Microsoft notes that how often it updates depends on the subscriber's provider.
Keep Outlook in sync with Fastmail automatically
Keeper.sh is a calendar sync service that copies events between Fastmail and Outlook, and between Google Calendar, iCloud and any CalDAV server as well. Each copy is a busy block named after the source calendar. The title, description and location are hidden by default, Pro can turn them back on per calendar, and guest lists are never copied. Each connection runs one way, so two-way sync is two connections; the calendar sync features page lists what else each connection can do. The Free plan covers 2 calendar accounts and 3 connections, with changes landing every 30 minutes. Pro is $5 a month or $45 a year, with Google and Outlook changes landing within seconds and iCloud, Fastmail and CalDAV changes landing every minute (pricing).
Setup takes a Fastmail app password and a Microsoft sign-in.
Create a Fastmail app password
Follow Fastmail's app password steps (checked 10 November 2026):
- In Fastmail on the web, open Settings > Privacy & Security.
- In Connected apps & API tokens, click Manage app passwords and access, then New app password. Fastmail may ask for your password first.
- Give it a name and leave access on the default Mail, Contacts & Calendars, which includes CalDAV. Files (WebDAV) does not cover calendars.
- Click Generate password and copy it.
Fastmail's Basic plan has no CalDAV access and cannot create app passwords, so this route needs a plan above Basic.
Connect Fastmail and Outlook in Keeper.sh
- In the Keeper.sh dashboard, open Add a calendar and choose Connect Fastmail.
- Enter your Fastmail Email Address, paste the app password into App-Specific Password and connect. There is no server address to type.
- Back in Add a calendar, choose Connect Outlook or Microsoft 365 and approve access on Microsoft's sign-in screen. Your Microsoft password never passes through Keeper.sh. Some Microsoft 365 organisations require an administrator to approve outside apps first.
- In the setup screens, answer Where should 'Calendar' send events? for the Fastmail calendar by ticking the Outlook calendar that should receive the copies.
Two-way sync between Fastmail and Outlook
For two-way sync, add a second connection from Outlook to Fastmail alongside the Fastmail-to-Outlook one. It sends Outlook events to a Fastmail calendar and is set up from the Outlook calendar's Send Events to Calendars list or the Where should … pull events from? setup screen.
If the goal is that colleagues booking you in Outlook see personal Fastmail appointments as busy time, the Fastmail-to-Outlook connection does that alone. Add the reverse connection when Fastmail should also show your work meetings.
Fastmail is one account and Outlook is the other. A two-way pair uses 2 accounts and 2 connections, which fits the Free plan with one connection spare. Pro removes the account and connection limits, and plans and limits lists what counts as each. For the same setup with Google Calendar as the destination, see syncing Fastmail Calendar with Google Calendar.
What a copied Fastmail event shows in Outlook
A doctor's appointment on a Fastmail calendar called Personal reaches Outlook as a busy block titled Personal at the same time, with no description or location, so a work account never sees the private title.
On Pro, Sync Settings on the Fastmail calendar turns on Sync Event Name, Sync Event Description and Sync Event Location one by one, and Exclude All Day Events leaves all-day events out (what a copied event shows). Because guest lists stay behind, no invitations go out from a copy and no one can join a meeting from it.
Copies carry a keeper.sh category in Outlook, which is how Keeper.sh recognises the events it created. Removing that category by hand makes Keeper.sh stop recognising the event as its own copy, so it leaves the event in place. Keeper.sh copies events from one month back to two years ahead by default, and Pro can change the range (how far events are copied).
How fast changes reach Outlook on each plan
On Pro, Fastmail changes land every minute. On Free, changes land every 30 minutes. On Pro, Outlook changes on a reverse connection land within seconds.
For how Keeper.sh compares with other sync tools on price and speed, see the best calendar sync tools compared. The same approach covers Apple's calendars in syncing iCloud Calendar with Outlook.
Fixing common Fastmail and Outlook connection errors
- Fastmail needs an app password here, not your Fastmail password. Fastmail refused the password. Usually the regular Fastmail password went in instead of an app password, or the account is on Basic, which has no CalDAV.
- Microsoft refuses the sign-in on a work account. The Microsoft 365 organisation blocks outside apps until an administrator approves them. Without approval, the Fastmail subscription link above still works in Outlook on the web.
- Copies in Outlook show the calendar name, not the event title. That is the default. Turn on Sync Event Name in Sync Settings on Pro.
- One repeating Fastmail event never arrives. Keeper.sh skips a repeating series with more than 10,000 occurrences inside the sync window, such as an event that repeats every hour with no end date. The rest of the calendar keeps syncing. End the series or make it repeat less often, and it syncs.
- Outlook changes never reach Fastmail. Only the Fastmail-to-Outlook connection exists. Add the reverse connection.
Questions about syncing Fastmail and Outlook
Can Outlook add a Fastmail calendar account?
No. Outlook does not support CalDAV, the protocol Fastmail calendars use, so on its own it can only subscribe to a read-only Fastmail calendar link.
Will my Fastmail appointments block time on my Outlook work calendar?
Yes. Keeper.sh copies Fastmail events onto the Outlook or Microsoft 365 calendar as busy blocks named after the Fastmail calendar. A subscribed link shows in Outlook as a separate, read-only calendar instead.
Can I edit a Fastmail event from Outlook?
No. A subscribed Fastmail calendar is read-only in Outlook, and a copy made by Keeper.sh is rewritten from the Fastmail event on the next sync. Edit the event in Fastmail.
Does calendar sync work on Fastmail's Basic plan?
No. Fastmail's Basic plan has no CalDAV access and cannot create app passwords, so Keeper.sh needs a Fastmail plan above Basic to read the calendar.
Does my Microsoft 365 organisation have to approve the connection?
Some do. A Microsoft 365 organisation can block outside apps until an administrator approves them. Without approval, a Fastmail subscription link still works in Outlook on the web.