We recently added some event sinks to an existing server. These sinks run fine on several other servers but are periodically causing EXOLEDB errors on this server. What's going on?
The errors are probably the result of a missing SystemMailbox mailbox. The proper name for this object is SystemMailbox plus a globally unique identifier (GUID) that matches the server GUID. Every server has one of these objects, which is linked to a disabled user object in the Microsoft Exchange System Objects container. To see the object, run the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in, then use the View, Advanced Features command. The mailbox contains a bunch of mostly undocumented system stuff, including registrations for Store-wide events and the global schema for the mailbox store, so its presence is required to avoid the errors you're running into.
The Exchange System Attendant will automatically recreate a missing SystemMailbox object if the user account is present, but if the user account is missing, the System Attendant can't find the correct account to which to link the mailbox—and you'll see the errors you describe. The Microsoft article "Event sinks do not function correctly if the SystemMailbox\{GUID\} mailbox is absent from an Exchange mailbox store" (http://support.microsoft.com/?kbid= 316622) describes two methods for fixing the problem. I recommend you take the simplest route, which is to create a new mailbox store, move all mailboxes to it, then delete the original store.
