When the data arrives from the remote client, the block lifts, and the thread resumes regular operation. The problem can be infuriating for administrators because the problem can affect one client but not a similar client that is connected on the same LAN at the same time. A temporary network condition might well block the remote client and prevent it from transmitting to the server if this blockage occurs, clients seem to hang. The problem occurs when the thread waits for additional datagrams to arrive from a client before it moves on to respond to other clients. NT provides an RPC Receive Any Thread service that the Exchange Information Store (IS) service uses to process client requests. The Microsoft article explains that MAPI clients that connect to Exchange over slower connections have a higher latency in sending and receiving datagrams (the basic building blocks of RPCs). If you want to keep phone charges to a minimum, you might want to use Outlook Express to check mail when you're on road trips.
I don't intend this observation to slam MAPI but to prove that the MAPI/RPC combination can demand a lot of network. Although your mileage will vary, inevitably you'll find that Outlook Express connects to Exchange Server faster and uses fewer network resources to do the same job. Then connect to another client such as Outlook Express 5.0 over either POP3 or Internet Message Access Protocol 4 (IMAP4), and repeat the same action. Connect a MAPI client, and send a simple message. Connect to a server using DUN, and monitor the number of bytes being sent and received. You can run a simple test to verify that MAPI clients use more resources. MAPI clients use more network resources than other clients in part because they provide extra features (e.g., calendaring) but also because they depend on RPCs. In most instances, if I leave the client alone for a little while, everything returns to normal.Įvery MAPI client from Capone, the original client shipped with Exchange Server 4.0 in 1996, through Outlook 2000 uses Windows remote procedure calls (RPCs) to communicate with the server. Outlook also hangs occasionally when I change folders or views on a folder. Most of the time you can send and receive mail without a problem, but occasionally Outlook 2000 appears to hang when I try to reply to or send a message. If you travel like I do, you spend quite a lot of time connected over RAS links back to an Exchange server. The root of the problem apparently lies with clients that connect over slow links. The Microsoft article "Exchange Clients Appear to Intermittently Hang During Normal Operation" () discusses the problem, but the details in the article are a little sketchy. Microsoft posted a hotfix for Windows NT 4.0 Service Pack 4 (SP4) for a problem that can cause Exchange Server to stop responding with all Messaging API (MAPI) clients.
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