How to prevent calendar invites from external sources automatically appearing in calendar globally

Hi Kloubek Michal, 

Thank you for posting your question in the Microsoft Q&A forum.  

Currently, Exchange Online does not offer a native tenant-wide setting to disable automatic calendar processing. However, with the PowerShell cmdlet shared earlier, you can extend it to perform a bulk update across all mailboxes in your tenant.

Here’s how you can do it:

Get-Mailbox -ResultSize Unlimited | ForEach-Object {
    Write-Host "Disabling auto calendar processing for:" $_.PrimarySmtpAddress
    Set-CalendarProcessing $_.Identity -AutomateProcessing None
}

This command loops through every mailbox and applies the AutomateProcessing None setting, effectively disabling automatic calendar handling such as auto-accepting or tentatively adding meeting invites. The execution time depends on the number of mailboxes—typically a few minutes for small tenants and longer for larger environments.

Additionally, Microsoft’s article Use mail flow rules to automatically add meetings to calendars outlines how mail flow rules can be used to manage calendar invites. While these rules cannot block calendar invites entirely, they can be configured to detect meeting requests from external senders and take actions such as quarantining, redirecting, or deleting the message. However, this method may impact legitimate calendar traffic, so it’s recommended to implement allowlists for trusted domains or senders.
I hope this help! If you have any further questions, please feel free to let me know.

Best regards,

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