No invitation confirmation sent for some invitations accepted through Thunderbird/Lightning

I googled a bit but could not find a description or explanation of the problem I observe:

We’re using NextCloud 18 for calendar (CalDAV) and a current Thunderbird for eMail. The NextCloud calendars are configured as CalDAV calendars in Lightning.

Another party is sending us event invitations from MS Outlook / Exchange.

Accepting the invitation works fine. Also forwarding the invitation mail to another user of our NextCloud works fine.

However the direct receipient or the user the invitation was forwarded to accept the invitation, no confirmation mail appears to be sent (I even checked the mail relay logs) out to the originating Exchange system.

I would strongly expect that both invitation accepts (of the original receipient and of the second user in this NextCloud instance to whom the invitation was forwarded) would trigger a confirmation mail to the inviter.

Does anyone else here experience this or is even able to explain what’s going wrong?

This sounds similar to the issue that I have with eM Client when connected to a Nextcloud calendar (currently running version 19.0.4 but all versions going back at least a couple of years show the same behaviour).

What I have established with the eM Client support team is that when a meeting invitation is accepted or declined the negotiation with CalDAV includes calendar-auto-schedule which means the sending of the response email is delegated from the mail client back to Nextcloud.

As far as I can tell, my Nexctcloud instance isn’t sending those emails out. I haven’t found a configuration option that makes it work either.


I found no solution so far and created an issue on GitHUB.

No comments there so far, but at least anyone interested can subscribe to it / monitor it.