I am running Thunderbird 115.11.1 on Windows 11 and no matter what I do I simply cannot send a calendar invitation. I click to create a new event, I add the email addresses of the attendees, I make sure to click the checkbox to send out invitations and as soon as I click Save and close nothing happens other than the addition of the event to my own calendar.

Yes, I have checked whether my email address is included in the calendar settings. Additionally, there is nothing in the Sent folder after clicking Save and close.