[opinsky]

How would it work for email invitations automatically being added as calendar entries?

[synchrone]

There is a message-based interoperability protocol for iCalendar: https://datatracker.ietf.org/doc/rfc5546/ and https://datatracker.ietf.org/doc/rfc6047/.

This is what Google Mail + Calendar are using, including features like RSVP, etc. It's all there, just passing iCal objects around.

[Rjevski]

No open-source calendar server that I know of supports this - they all rely on your email client managing this client-side (when you accept an invitation it adds it to your calendar).