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On a Microsoft Exchange Server server that is running Microsoft Forefront Security for Exchange Server, you may notice mail queues start to form. If you perform a Performance Monitor analysis, you may notice lots of Lightweight Directory Access Protocol (LDAP) queries.
Remote procedure call (RPC) threads may become unavailable if there are lots of LDAP queries. The unavailability of RPC threads causes e-mail messages to queue up.
To workaround this problem, disable Content Filtering for the Realtime scan job. To do this, follow these steps:
In the Forefront Server Security Administrator client, click Operate, and then click Run job.
Click Realtime Scan Job, and then click to clear the Content Filtering check box.
To resolve this problem, install Forefront Security for Exchange Server with Service Pack 2 (SP2).
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
960465 Description of Forefront Security for Exchange Server with Service Pack 2
FSE email queues stop working RPC LDAP queue Forefront Exchange