Why are people having trouble posting on our Shared Calendar in Outlook?

Question

Note one person at the bottom of the attached screenshot says she is outside of our organization but she is not. She has a full subscription to Microsoft 360.

Then today, the person at the top with email address of [Moderator note: personal information removed] -- who has always been able to post meetings in the calendar, suddenly is getting a message saying "The set of folders cannot be opened."

Can you please help?

Answer 1

Hi @Erin O'Brien,

Welcome to Microsoft Q&A forum.

Thank you for reaching out and sharing information regarding a user cannot post an event to a shared calendar and the other one with Microsoft 365 subscription is recognized as an external user of the organization.

To move forward securely, please check the private message session I have initiated with PII related: 

1. In your thread, you will see a banner regarding ongoing private message. 
2. Click View messages. 

Thank you for your patience and cooperation. I'm looking forward to your reply. 

Answer 2

Trouble posting to a shared calendar in classic Outlook is usually caused by one of these issues:

1. Calendar sharing/permissions problems

• Verify that the shared calendar is still shared correctly and that affected users have at least Reviewer or Editor permissions.
• In classic Outlook, the calendar owner should:
  1. Go to Calendar.
  2. On the Home tab, select Share Calendar and choose the shared calendar.
  3. In Calendar Properties > Permissions, check each user:
     • Confirm the correct email address is listed (not a group address).
     • Set an appropriate permission level (for posting, typically Editor or higher).
  4. If needed, remove the user and Add them again, then re‑send the sharing invitation.
• If someone is shown as “outside the organization” even though they are not, this is controlled by the organization’s sharing policy. An admin may have restricted external sharing or the user’s account may be in a different tenant/domain than expected. Only an admin can change those sharing policies.

2. “The set of folders cannot be opened” for a shared calendar This error usually indicates a problem with the shared calendar entry in the user’s Outlook profile rather than with the mailbox itself.

For the affected user (e.g., ******@berkshirehabitat.org):

1. Remove and re‑add the shared calendar
   • In Calendar, right‑click the problematic shared calendar and choose Delete Calendar (this only removes it from the profile, not from the owner’s mailbox).
   • Close and restart Outlook.
   • Have the owner re‑share the calendar (as above) and send a new invitation.
   • The user should open the invitation and select Accept so the calendar is added back.
2. Turn on shared calendar improvements (if using Exchange Online)
   • In Outlook: File > Account Settings > Account Settings.
   • Select the Microsoft Exchange account and choose Change….
   • Select More Settings > Advanced.
   • Check Turn on shared calendar improvements, then close and restart Outlook.
   • Have the calendar re‑shared and re‑accepted if necessary.
3. Check organization‑level sharing policies (admin)

• If some users are flagged as external or cannot be granted the needed permission level, an Exchange or Microsoft 365 admin should review the organization’s calendar and external sharing policies and adjust them if internal users are being treated as external.

If, after these steps, users still cannot open or post to the shared calendar, the next step is to use the Outlook support options (Help in Outlook or the Microsoft 365 admin center) to collect logs and escalate, as this can indicate a backend mailbox or directory issue.

---

References:

• Share an Outlook calendar as view-only with others
• How to enable and disable the Outlook calendar sharing updates
• Sharing calendar and contacts in Microsoft 365
• Known issues with shared calendars in Outlook
• Microsoft 365 group users may be unable to view or access group calendars and email messages in Outlook Desktop