User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20171201053558 Steps to reproduce: I have one calendar configured, it is CalDAV (DAViCal server), all on Debian stable (stretch) I sent an invitation to some people. It appears in my calendar and in the Sent folder (IMAP) but the recipients didn't receive it. In another case, I received an invitation from somebody and clicked accept. The reply appears in the Sent folder and the meeting is in the calendar but the other person didn't receive the reply. Actual results: In both cases, Lightning didn't display any errors or warnings. It looked like everything happened successfully and I only found out later that the other people don't receive anything. Expected results: either the other people should receive the invite/acceptance or I should see an error message telling me something went wrong and telling me to try again.

I noticed some related bugs but I don't think they are the same, e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=1397767

Check your server settings. Davical (like some other caldav servers) promotes by deafult to the client the autoscheduling feature about dealing with invitations serverside which expectedly prevents Lightning from doing email scheduling. If you disable it in the server configuration, Lightning will do email scheduling. Starting with the upcoming ESR, you will have the ability to overrule a caldav server's autoscheduling feature within Lightning in the respective calendar properties dialog (bug 1299610).