In previous versions of Exchange, the System Attendant (SA) Mailbox was used by the link monitoring service and for publishing Free / Busy information.  However, this functionality is no longer needed in Exchange 2010.  As such the mailbox has been removed.  One catalyst to this change was the new High Availability (HA) features in Exchange 2010.  Having the System Attendant mailbox on servers presents a problem when the server needs to failover to the passive mailboxdatabase copy.  Since only one SA mailbox can exist per server which SA mailbox do you use? The mailbox database's passive copy or the one from the mailbox that just failed over?

As pointed out in Evan's blog post (See Additional Resources), there are two components to the System Attendant mailbox, the directory object and the mailbox object.  The key here is that the directory object has _not_ been removed, only the mailbox object.  Therefore, you will still be allowed to connect to Exchange using the System Attendant however you will not be able to logon (effectively open the mailbox object).

So what are your workarounds?

1. You can still use the per mailboxdatabase System mailbox (See Additonal Resources for more information about this mailbox).

2. You can change your application to rely on a user created account and mailbox.

Additional Resources
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Exchange Special Mailboxes - Part 1 System mailbox

http://blogs.technet.com/evand/archive/2004/12/17/323636.aspx

Exchange Special Mailboxes - Part 2 System Attendant Mailbox

http://blogs.technet.com/evand/archive/2004/12/21/329172.aspx

Exchange 2010 High Availability

http://technet.microsoft.com/en-us/library/dd335211(EXCHG.140).aspx

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