Outlook calendar sync not working in your ATS: causes and fixes

When you have Outlook calendar sync not working in your ATS, the sign is usually a mismatch. An interview booked in the ATS doesn't show up in Outlook. Or a meeting added in Outlook leaves a slot open for candidates to book. This guide covers what the symptom means, the common causes behind Microsoft 365 calendar sync problems in recruiting, and the steps to fix them.

What the symptom means

In ATS Mako, calendar sync keeps availability accurate in both directions. A slot booked elsewhere shouldn't stay open for a candidate to double-book. Candidates self-schedule against real availability, and booked interviews land on a shared calendar that recruiters and hiring managers work from. When sync breaks, one direction usually stops. That tells you where to look.

• Interviews candidates book through your link don't appear in Outlook.
• Meetings added directly in Outlook don't block time on your booking link.
• Interviews appear in Outlook at the wrong time.
• Sync works for one recruiter but not for another.
• Sync worked for a while, then stopped with no obvious change.

Likely causes

Permissions and consent issues

Apps reach Outlook calendar data through Microsoft Graph, using permissions that a user or an administrator approves. Your organization's Microsoft 365 admin can turn off the ability for staff to approve apps themselves. In that case, an admin has to grant consent for everyone. Admins can also set custom policies that block access even after consent, which can show up as an access denied or 403 Forbidden error.

Shared and delegated calendars are a common trap. Microsoft notes that delegated permission only supports subscribing to items in the signed-in user's own mailbox, and that sharing permissions don't support change notifications on shared or delegated folders. If a recruiter connects their own account but expects a coordinator's calendar or a shared team calendar to sync, updates may not flow.

Change notification delays and stopped connections

Apps stay in step with Outlook by subscribing to change notifications for events in a user's calendar. Those subscriptions expire and must be extended. Microsoft also describes lifecycle notifications, which exist because an app can have its subscriptions removed or miss some notifications. A gap here looks like late updates, or sync that worked and then stopped.

There is also a ceiling on subscriptions per mailbox shared across every app. Microsoft states there is a maximum limit of 1000 active subscriptions for Outlook resources per mailbox for all applications. A mailbox connected to many tools is worth flagging to support.

Time zone mismatches

By default, the Outlook calendar API returns event start and end times in UTC and records the original time zones separately. If the time zone on a user's Outlook calendar doesn't match the time zone set for their office, interviews can look shifted. ATS Mako lets you set a timezone per office for apply pages and the candidate offer portal, so check that it matches.

Fix steps

1. 1 Reproduce the problem with a test Book a test slot through your booking link. Then add a test meeting directly in Outlook. Note which direction fails and which users are affected.
2. 2 Confirm the account and calendar Check whether the connected account is a work or school account or a personal Microsoft account. Make sure the calendar you expect to sync is that user's own calendar, not one shared with them.
3. 3 Check consent with your Microsoft 365 admin Ask your admin whether user consent is turned off or custom access policies apply. If so, the admin may need to grant consent on behalf of users before sync can work.
4. 4 Connect each interviewer's own calendar If you rely on shared or delegated calendars, have each recruiter or interviewer connect their own calendar instead. Availability and booking work across every interviewer on a requisition.
5. 5 Check time zones Compare the time zone on each affected Outlook calendar with the timezone set for that office in ATS Mako. Fix whichever is wrong.
6. 6 Report a stopped connection If sync worked and then stopped, the notification connection may have lapsed. Send support the details listed below so the connection can be checked.
7. 7 Clean up affected interviews Once sync is back, use bulk event updates to reschedule or retime any batch of interviews that were affected, instead of editing them one at a time.

How to confirm it worked

• A new test booking appears on the interviewer's Outlook calendar.
• A meeting added in Outlook blocks that time on your booking link.
• Interview times match in both places.
• Every interviewer on the requisition passes the same check, not just the person who reported the issue.

For more on how booking and sync fit together, see the overview of interview scheduling and calendar features.

When to contact support

Contact support if the steps above don't fix it, if you see an access denied error after your admin granted consent, or if sync stopped without any change on your side. Include the details below so the issue can be traced quickly.

Affected user email(s):
Account type (work/school or personal):
Direction that fails (ATS to Outlook, Outlook to ATS, or both):
When sync last worked:
Example interview (candidate, date, time):
Any error message shown:
Has your Microsoft 365 admin granted consent? (yes/no/unsure):
Is the calendar shared or delegated? (yes/no)