Internet searching 101 (STFW): Google for ical and javamail. Among the top results is Sun's documentation for their iCalendar server.

Sounds like email ("iMIP", RFC 2447) is one of the alternate transports available (HTTP is another), underneath the transport independent calendar exchange protocol ("iTIP"), to exchange iCalendar information between iCalendar servers.

So presumably you would use JavaMail underneath to send and receive messages, with an added API on top for the iCal specific details inside those messages.

But Sun discourages using email, in favor of HTTP instead, for this purpose, since the email transport system is not designed for this -- email can easily become overloaded by the exchange of so many frequent small messages.

Also other results from the Google search suggest that "CAP" is going to be a better way to go, than ITIP/IMIP, as the calendar exchange protocol, but it is still not finalized yet.