On an Exchange 2010 mailbox server, the Information Store service may become unresponsive and eventually dismount and/or be forcibly terminated. When this problem occurs, Exchange users keeps receiving an RPC error that Outlook is trying to contact Exchange. If you use Performance Monitor to view the values for Version Buckets Allocated under MSExchange Database -> Instances, you will find that the value increases constantly for one or more databases until those databases eventually fail.
Event 10027 may be logged when this problem occurs.
Event 10027 may be logged when this problem occurs.
Event ID: 10027 Source: MSExchangeIS Description: There are 5 RPC requests for the mailbox " <User> " on the database " <Database> " that have taken an abnormally long time to complete. This may be indicative of performance issues with your server.