In an Exchange environment the BES acts as a MAPI (Messaging Application Programming Interface) client. During the start of the BES its MAPI client component gets a referral from Exchange to a Global Catalog for address book lookup calls.
In a situation where the Global Catalog being used by the BES goes offline (or fails) you may experience problems sending email and doing address lookups from the Blackberry Device until you restart the BES services.
When using a “normal” MAPI client, like Outlook, outage of the Global Catalog being used in the clients MAPI sessions would only cause temporary issues. The MAPI client gets a referral to a different Global Catalog (GC reconnect logic was added to Exchange around 2004) and regains full functionality. However when your MAPI client runs as a service the referral process breaks down. When this happens the only way to recover is to restart the MAPI client. In the case of a BES this means restarting the BES services.
There is a workaround to prevent the referral process from breaking down on the BES. Globally it means making adding a related setting (EMSAB_UserAuth_Credentials = 1) to the registry.
Details are documented in KB16118 for different setups of the BES. KB25066 describes testing if the change works in your environment. More information about why the referral process breaks down can be found in Microsoft KB929439.