How can I stop external calendar (unresponded) events from showing?

Dear @Dominik Weber,

Thank you for posting your question in the Microsoft Q&A forum. I understand your concern about unwanted external calendar events showing up in your Outlook calendar and you cannot delete those events without response to customer in the New Outlook.

Based on what you've provided, I recommend some steps below:

1/ Use Classic Outlook
Classic Outlook still provides the option to Delete without sending a response, which is missing in New Outlook.

• Open the event in Classic Outlook > Click Delete > MS Outlook will ask you whether you would like to respond to the meeting organizer, then select "No".
• This prevents notifying the sender and avoids confirming your email address.
  

2/ Create mail flow rule to block the similar cases.

• Ask your IT admin to create mail flow rules to block or quarantine meeting requests from suspicious external domains. For reference:
  • Upgrade the security level for the executive security policy Authentication settings from the admin portal: Configure spam filter policies - Microsoft Defender for Office 365 | Microsoft Learn
  • If the email contains specific common words or phrases in the subject line or sending I.P, you can use Transport rule (Mailflow rule) to block invitations. Please refer to the attached link on how to create mailflow rules : Manage mail flow rules in Exchange Online | Microsoft Learn
  • Report the email as spam to Microsoft from the admin portal, check this : https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/admin-submission?view=o365-worldwide on how to do submission in portal

Please understand that our initial response does not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.

If I misunderstood what you’re looking for, feel free to let me know or share a screenshot. I’d be happy to help further! 

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

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https://learn.microsoft.com/en-us/answers/questions/5597205/prevent-pfishing-calendar-items

Hal Hostetler MVP-Outlook MVP Volunteer Moderator Oct 26, 2025, 12:21 PM FWIW, this issue has been reported to Outlook Engineering, they are aware of the problem, and are investigating it.

SPAM Calendar Events - Update 2025-12-05 - Still an ongoing issue. No update from Microsoft on a general solution.

If your organization has a Microsoft Defender for Office 365 subscription, there are steps your IT Admin can take to mitigate the impact:

https://techcommunity.microsoft.com/blog/microsoftdefenderforoffice365blog/strengthening-calendar-security-through-enhanced-remediation/4456876

If you are in an IT Managed environment, there are Mail Flow Rules that your IT Admin can implement that will block all external Calendar invites (and allow for specific exceptions):

https://learn.microsoft.com/en-us/answers/questions/5558001/how-to-prevent-calendar-invites-from-external-sour