iCalendar allows to add attendees to an event. The attendee receives a special invitation email.
There are basically three ways to send this mail.
Invitation sent by server
Many servers support sending invitation mails. This is the preferred way because the server is able to manage invitations and answers. The server can update the status of the event when receiving an answer from an attendee and directly insert it into the calendars of internal attendees. Furthermore, the mail configuration has only to be done once.
Invitation sent by calendar user agent
There are also calendar apps that support invitation mails. Big advantage: The invitation can be sent with a custom text.
Invitation sent by sync adapter
There are two possibilities of implementing emailing into the sync adapter.
- Sending the mails directly via SMTP. That would mean to built in an SMTP client and an IMAP client (to put the mails into the sent folder) and thus additional configuration effort for each account on each device.
- An alternative is to call an android mail app to send the mails manually. Unfortunately we don't know if it's possible to forward the necessary headers properly and if mail apps support this special kind of mails at all.
The main problem is that CalDAV-Sync is running in the background and syncing periodically. Hence, it is not necessarily active at the moment you create an event. So the mail app would pop up at any point of time.