Invitations by e-mail do not work (CalDAV)

[Foddy]

Hi,

I've successfully created a calendar and can sync it with both Thunderbird and the Calendar app on a Mac. However, when I try to invite a person by email to accept an appointment, I get the following error on “Calendar”:

In Thunderbird, I can create the invitation and sync with the server. However, no e-mail will be sent to the invited person.

Poste.io is configured correctly and able to send normal mails. Does somebody has any idea? I came across this article here, unfortunately I do not know how to setup this in the Poste.io container or if that is necessary at all:

http://sabre.io/dav/scheduling/

Thank you in advance!