RPC clients or MAPI on the Middle Tier clients may not receive responses from the mailbox server role on an Exchange 2010 server

Under a heavy email workload environment, when users try to connect to their mailboxes on a Microsoft Exchange Server 2010 Hub Transport server by using a RPC client or by using a MAPI on the Middle Tier (MoMT) client (such as Microsoft Outlook, for example), the client may not receive responses from the mailbox server role. Additionally, there are lots of email messages that are stuck in delivery queues.

To resolve this problem, install Update Rollup 3 for Exchange 2010.
For more information about Update Rollup 3 for Exchange 2010, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.