In Reader to Reader: "SP3 and Third-Party Connectors" (December 2000), David Barnes discusses a problem that occurs with certain third-party connectors, such as Equisys's Zetafax or GFI Fax & Voice's FAXmaker for Exchange. The problem is that Outlook hangs when a client tries to delete, move, or copy multiple items. The solution that David presents is to run (or in some cases rerun) the Exchange Performance Optimizer.
I'm a senior technical consultant at Fenestrae, and I've encountered this problem on several occasions. In my experience, David's solution doesn't always solve the problem. I've found that in many cases the problem doesn't occur until after you run Performance Optimizer. The cause of the problem is that the Exchange Information Store (IS) runs out of background threads. Running Performance Optimizer resets the number of threads and therefore is just as likely to cause the problem as to solve it.
Manually setting a sufficiently high number (e.g., 62, or hexadecimal 3E) of available background threads always solves the problem. To set the number of available background threads, open regedt32, go to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\Parameters System registry subkey, and change the Background Threads value to at least 3E. For this change to take effect, you must stop and restart the Exchange Information Store service.