Thunderbird 113 (?), local calendar, Win 10, email invites stopped being sent to the listed Attendees. Correct email was entered on Calendar/Properties/Email and Notify attendees was ticked in New Event dialogue. Attendees were correctly entered under New Event/Invite Attendees but after clicking Save and Close, there was no Invitation email record in "Sent" emails and no Attendees receives email invitations. The event was correctly saved in the originator's calendar. TB 115.7.0 installed from scratch on a new Win 11 PC and the emails/calendar data transferred from the old PC. Everything works *except* that no invitation emails are still sent to the listed Attendees of a new event (same symptoms as on the Win 10 PC). So transferring emails and calendar entries transferred the problem to the new PC. What next?