When I get an invitation to an event in an email, Thunderbird identifies that it's an invitation and says "This message contains an invitation to an event" and displays buttons allowing me to "accept" or "decline". If I accept, the event is added to my calendar. In Thunderbird 38.3 and previous, Thunderbird automatically generated and sent emails to the invitor, so that their calendar program knew who had accepted, declined, and not responded. Since updating to 38.4, Thunderbird no longer generates these emails. It will still generate an email if you delete an accepted event from your calendar. Thunderbird is our company email client and this is very inconvenient. Thunderbird release channel on Windows 7. I have not tried to replicate this on other platforms.