All servers are running Windows 2008. During a test we shuted down a DC, not the PDC. Both DC ar GC. For a reason or another one Exchange server was having problems with the system attendant, so the mailboxes were unavailable.
I used "Modify configuration domain controller" to choose a DC. The everything came back. But as far as I know Exchange shouldn't have any trouble to run when a DC goes down, no matter which DC is down.
Here is what I found in the event viewer
Microsoft Exchange System Attendant failed to read the membership of the universal security group '/dc=ca/dc=ourdomain/ou=Microsoft Exchange Security Groups/cn=Exchange Servers'; the error code was '8007203a'. The problem might be that the Microsoft Exchange System Attendant does not have permission to read the membership of the group.
If this computer is not a member of the group '/dc=ca/dc=ourdomain/ou=Microsoft Exchange Security Groups/cn=Exchange Servers', you should manually stop all Microsoft Exchange services, run the task 'add-ExchangeServerGroupMember,' and then restart all Microsoft Exchange services.
Could not read the Security Descriptor from the Exchange Server object with guid=D922679FE5725A4BBA65373361CF9BF1. As a result the Proxy Address Calculation RPC interface will not be available on the local Exchange Server.
Error initializing session for virtual machine S2008MB001. The error number is 0x80040111. Make sure Microsoft Exchange Store is running. Also, make sure that there is a valid public folder database on the Exchange server.