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How to Enable Cross-Site Connectivity for POP3 and IMAP4 Clients

Microsoft Exchange Server 2007 will reach end of support on April 11, 2017. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.


Applies to: Exchange Server 2007 SP1, Exchange Server 2007 SP2, Exchange Server 2007 SP3

By default, POP3 and IMAP4 connectivity between a Client Access server in one Active Directory site and a Mailbox server in another Active Directory site is not enabled. The Client Access server prevents users from logging on to their mailboxes by using POP3 or IMAP4 if their mailboxes are located on a Mailbox server in a different Active Directory site.

This topic explains how to configure Microsoft Exchange Server 2007 to enable cross-site connectivity for your POP3 and IMAP4 clients.

Consider the following before you enable cross-site connectivity for POP3 and IMAP4 clients:

  • Your messaging system may be more secure if you do not enable it.

  • You may notice performance issues if you enable it. When this setting is enabled, the Client Access server will make a direct RPC connection to the user's Mailbox server in the alternate site instead of rejecting the connection. RPC connections from the Client Access server in one Active Directory site to the Mailbox server in another Active Directory site are expensive in bandwidth and resources when traveling over WAN links.

When a user tries to log on cross-site when cross-site connectivity is not enabled, event MSExchangePOP3 1008 or event MSExchangeIMAP4 1008 will be logged in the Application log of the Client Access server. The event description for these events is as follows:

  • User "<User Name>" was prevented connecting to his/her mailbox because the Mailbox Server resides in a different Active Directory site (Active Directory site name) than the Client Access Server (<Client Access server name>) to which he/she connected. Either arrange for the Client Access server and the Mailbox server to reside in the same site, or change the configuration setting of AllowCrossSiteSessions to true.

When you upgrade to Exchange 2007 Service Pack 1 (SP1), the cross-site connectivity setting will be set to the default setting. If you have enabled cross-site connectivity, and you do not want to reconfigure this setting after you upgrade to Exchange 2007 SP1, you can back up your POP3 and IMAP4 configuration before you upgrade to Exchange 2007 SP1 and then restore the configuration after you complete the upgrade. For information about how to do this, see POP3 and IMAP4 May Not Work as Expected After You Upgrade to Exchange Server 2007 SP1 or a later version.

Before You Begin

To perform the following procedures on an Exchange 2007 Client Access server that has the POP3 or IMAP4 service enabled, the account you use must be delegated the Exchange Server Administrator role and membership in the local Administrators group for the target server.

For more information about permissions, delegating roles, and the rights that are required to administer Exchange Server 2007, see Permission Considerations.

You must perform this procedure for each protocol for which you want to enable cross-site access.


To enable cross-site connectivity for POP3 and IMAP4 clients

  1. On the Client Access server, locate the POP3 and IMAP4 configuration files. By default, they are located at C:\Program Files\Microsoft\Exchange Server\ClientAccess\PopImap, where C:\ is the directory in which you installed Exchange Server 2007.

  2. Use any text editor, such as Notepad, to open the configuration file that represents the protocol for which you want cross-site connectivity. The two configuration files are as follows:

    • Microsoft.Exchange.Pop3.exe.Config

    • Microsoft.Exchange.Imap4.exe.Config

  3. Search for the key AllowCrossSiteSessions. By default, the line in the configuration file is as follows.

    <add key="AllowCrossSiteSessions" value="false" />
  4. Edit the line of the configuration file as follows.

    <add key="AllowCrossSiteSessions" value="true" />
  5. Repeat the steps in this procedure for each protocol for which you want to enable cross-site connectivity.

  6. Restart the services for each configuration file that you have updated to enable cross-site connectivity. The names of the services are the Microsoft Exchange IMAP4 service and the Microsoft Exchange POP3 service.

For More Information

For more information about how to manage POP3 and IMAP4 access for Exchange 2007, see Managing POP3 and IMAP4.

For information about how to plan for Client Access servers, see Planning for Client Access Servers.