If your Exchange clients randomly hang, for up to a minute, the RPC service on your Exchange server may be very busy.
The RPC service uses a synchronous mechanism to pick up messages. If you have slow clients, RPC may get temporarily stuck.
You should use Performance Monitor on RPC related traffic, such as:
MSExchangeIS\Write Bytes RPC Clients/sec--May drop to 0 briefly.
MSExchangeIS\Read Bytes RPC Clients/sec--May sometimes spike after the Write Bytes RPC have corrected themselves.
Process\Thread Count\STORE--Sometimes spikes briefly, as high as from 160 to 211, then corrected itself.
If your hardware can support it, you should continue to monitor activity as you increase the following registry entries:
Microsoft reports that on one customer's quad-processor, a Max of 200 (decimal radix) and a min of 100 proved successful.
Increasing the Minimum Threads allows the RPC service to handle more messages. Increasing it too much WILL HANG your server.
If you have more than 800 MB of RAM, see Q195006.