Meeting acceptances from external attendees do not reach OS X Server

Question

OS X Server allows you to run your own calendars, away from iCloud. So when setting up meetings and sending out invites, I would like to know, what the status of the people is I have invited.
Trouble is, that when an external attendee wants to confirm, OS X Server's mail function rejects the email as the recipient is unknown in the local recipient table:
From: Client <[email protected]>
To:
  [email protected]l.example.com
Subject: Event accepted: Lunch
Your message cannot be delivered to the following recipients:
Recipient address: [email protected]l.example.com
Reason: Remote SMTP server has rejected address
Diagnostic code: smtp;550 5.1.1 <[email protected]l.example.com>: Recipient address rejected: User unknown in local recipient table
In my view, Calendar and Mail should work together, so that responses can come through and can be processed by the Server, but without compromising security or allowing spam mail through.
Any ideas on how to achieve this?
Thanks for your ideas
LL

Answer 1

...and the com.apple.calendarserver user exists?  You can check using dscl in Terminal.
bash-3.2$ dscl
Entering interactive mode... (type "help" for commands)
> cd Local/Default/Users
/Local/Default/Users > read com.apple.calendarserver