I had this typical issue wherein a user who was no more with the Organization had left without canceling his meetings. Below are the details of the issue

Description:
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User A sent a meeting request to 20-30 other users, reoccuring once every month. Later User A left the Organization and did not cancel the meeting. The user account was deleted, but the mailbox was still available, even after reconnecting the mailbox, this meeting request was invisible in Calendar, Deleted Items, Inbox, etc...

Cause:
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There can be numerous reasons why this meeting request was unavailable, one of the possible reason was to delete the meeting request without cancelling the meeting

Resolution:
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Ran the powershell to delete the calendar item with specific keywords in the subject and from a specific sender from any Calendar in the Organization. Below is the script:

Get-Mailbox | Export-Mailbox -SenderKeywords <sendermailnickname> -SubjectKeywords "enter keywords here" -IncludeFolders "\Calendar" -DeleteContent

The above command will search every mailbox in the Organization and delete the meeting request meeting both the conditions mentioned

If you want to delete the meeting request from a particular users' calendar, you can use:

Get-Mailbox | Export-Mailbox -Identity <users' mailnickname> -SenderKeywords <sendermailnickname> -SubjectKeywords "enter keywords here" -IncludeFolders "\Calendar" -DeleteContent

If you want to test if the rules are matching the required criteria before deleting, you can export the item to another test mailbox, using:

Get-Mailbox | Export-Mailbox -Identity <users' mailnickname> -TargetMailbox <testmailbox> -SenderKeywords <sendermailnickname> -SubjectKeywords "enter keywords here" -IncludeFolders "\Calendar"

You can also use -TargetFolder to export the information to a specific folder and the outcome will look like -