Msexchangefbpublish and error updating busy
I want to also point out here that the availability service also runs the calendar assistant which does things like automatically accept meeting requests in a tentative state, while waiting for your reply, hence writing data to your calendar under your mailbox on your behalf.
Mail does send and receive though so the error message is more of an annoyance.
Outlook publishes the free/busy data for a user once every 15 minutes and again when Outlook is being shut down.
Every message includes the entire free/busy information for the corresponding user that ranges from the current month to two months in the future.
If it needs to talk to a mailbox server in another site, it will relay the request via HTTP/HTTPS to the client access server in that site, then to the mailbox server via MAPI.
Remember if you are going to put exchange in a remote site you always need client access, hub transport and mailbox role at least.
To resolve this problem, reset the system article on how to do this: