XIMS: MCIS IMAP4 Mail Server May Exhibit Intermittently Poor Performance (265389)

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SYMPTOMS

Microsoft Outlook Express clients that are connecting to a Microsoft Commercial Internet System (MCIS) IMAP4 server may experience poor performance or time-out errors. The responsiveness of the data store for the IMAP server may be slow.

If MCIS 2.5 HTML Mail Interface (HMI) is configured to access the IMAP server, Web clients that access their mailboxes may experience poor performance or a time-out error. ASP 05 errors may be logged on the HMI server, and ASP 0115 errors may be returned to the Web clients.

CAUSE

The Microsoft Windows NT redirector is not choosing an optimal network path. If the IMAP server is configured as a multi-homed system, the redirector has multiple paths available for accessing a remote Windows NT server that is acting as the mail store. When the primary network bound to the adapter provides a connection to the mail store over a slow network route, this route is likely to be used if it is successfully connected. Slow network routes may be caused by multiple default gateways, as when one of the network interface cards (NIC) is a Dynamic Host Configuration Protocol (DHCP) client and another NIC has a static default gateway.

In a Network Monitor trace of the traffic between the IMAP4 server and the mail store, the IMAP server may send an Address Resolution Protocol (ARP) broadcast looking for an alternate route to the mail store. If the mail store replies over a different route, the Network Monitor trace shows that the mail store is replying from a different address.

RESOLUTION

Provide the IMAP server with a high-speed back-end network connection to the Windows NT mail store server. If the network adapter on the IMAP server is attached to this back-end network and it is the primary adapter, this path is used by the redirector unless a connection cannot be established over this path. Other network routes are only used if a connection request doesn't work.
Attach the back-end network to the IMAP server by means of a network adapter that is not the primary bound adapter. If there are no other network routes available to connect to the mail store, the IMAP server should consistently connect by means of this route.
If the primary network adapter can connect by means of a valid but less-than-optimal route, it does so. To avoid this behavior, configure the redirector with the ignorebindingorder option, as documented in the following Microsoft Knowledge Base article:
166159 NetBIOS Connections from a Multihomed Computer

NOTE: If the IMAP server is configured with Windows Load Balancing Service (WLBS), as described in the following Microsoft Knowledge Base article,

197991 How to Configure WLBS Using Multiple Network Interface Cards

the primary network adapter may not be the back-end network. If this is the case, the best option is to assure that the back-end network has absolutely no available route between the public WLBS network and the private back-end network where the mail store resides.

MORE INFORMATION

To determine if the IMAP server is receiving a delayed response from the mail store because of network-related problems, perform the following command from the console of the IMAP server

dir \\servername\mailstore

where servername is the name of the server experiencing the problem.

NOTE: Name resolution can also be verified by a PING server. To assure that there are no name resolution-related problems in accessing the mail store, use the following command

dir \\ipaddress\mailstore

where ipaddress is the IP address of the server experiencing the problem.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

248821 How to Improve IMAP Fetch Response Time with Remote File Server[sitemcis]