I noticed this when responding to calendar invitations from folks with Google accounts. Google sends me a bounce message rejecting the calendar acceptance notifications because they are not signed with DKIM, since my DMARC settings require all messages to be signed with DKIM. However, the recipient still sees my calendar response in their Google calendar, which is interesting. I checked my sent messages and was surprised to see that the calendar acceptance from Thunderbird is sent without the normal DKIM signature configured on my email server. I run Postfix with port 587 for sending email using the submission protocol, and Dovecot on port 993 (IMAP/S). I may be able to set my Postfix installation to sign the calendar notifications with DKIM using a header check, however, that requires some advanced configuration and I'm not really sure it's possible-- I also noticed that my normal header checks do not seem to trigger when sending these calendar notifications. My question would be why Thunderbird acceptances/declines are not sent through the normal SMTP channels that would trigger a DKIM signature. I didn't see any configuration options for how acceptances and declines are generated, or how to send them.