OS X Server: iCalendar Message-Based Interoperability Protocol (iMIP) support
The Calendar service in OS X Server can support email invitations via iMIP (iCalendar Message-Based Interoperability Protocol) that require the server to have access to an IMAP server for incoming mail and an SMTP server for outbound mail.
Lion Server, OS X Server (Mountain Lion)
The incoming IMAP email service must support "plus addressing", the addition of a plus (+) sign and a tag to the name portion of the email address to create a sub-address which the Calendar Server uses to recognize an invitation reply.
If invitation responses are not being acknowledged by the Calendar Server, please check with your ISP (Internet Service Provider) to verify that “plus addressing” is supported through their mail servers.
Note: The IMAP account used for this purpose should be dedicated solely to the calendar server; other incoming emails will be discarded.
For details about "plus addressing," see http://tools.ietf.org/html/rfc5233#section-3.
For more information about iMIP, see http://trac.calendarserver.org/wiki/FAQ.