I see versions of this question have been asked and answered before. Unfortunately, none of the suggested fixes are successful. I apologize if I have missed the solution. I use Google calendar and have a private domain email address (e.g. dw@dwan.com). I have Thunderbird Calendar set up properly; it successfully displays all my Google calendar appointments, and appointments entered in either system shows up in the other. The primary calendar that is displayed in Thunderbird Calendar is my Google calendar (called gcal) -- I have deleted the default calendar, so the only one displayed is my Google calendar. When someone sends me an invitation, I can click Accept, and it adds it to my calendar, no problem. However, no email response is sent, and the inviter's calendar never gets updated to show my acceptance. I have tried clicking Accept as well as Send Response Now; the results are the same. I have checked the configuration and set calendar.google.enableEmailInvitations and calendar.itip.notify to True. Any ideas? Is this because the email receiving the invitations is not a gmail account? Is it related to my having deleted the default calendar? It's embarrassing that business contacts need to check with me whether I am coming to an event I have already responded to. Thank you in advance for any help! Thanks! Dwan