Mail Source Agent failure during the processing of a Remote User Collection may cause mailbox lock (4285187)

A Mail Source Agent (MSA) may experience an RPC failure during the processing of a Remote User Collection (RUC) generating a log entry of the form:

03.03.2012 09:30:38 RpcStoreAdmin::InitEx Error -2147221231 ClassFactory cannot supply requested class : CExchAttendantCalls::ExAtOpenStore failed File: 'aeRpcConnection.cpp' Line: '90'

03.03.2012 09:30:38 RpcStoreAdmin::SetInTransit Error 91 Object variable or With block variable not set

On the Microsoft Exchange Server, you may see application event log ID 9660 warning that the user "failed to log on because their mailbox is in the process of being moved". Users with mailboxes that were being processed at the time may remain locked out and unable to logon to their mailbox.