Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool reads the following registry entry to determine if the following registry value exists:


If the Exchange Server Analyzer finds the Compatibility value present and configured with a value of 0xfffffffe (in hexadecimal), a non-default configuration message is displayed.

Exchange Server Internet Protocol (IP) client computers that are using IMAP4 and Post Office Protocol version 3 (POP3) fully use the streaming store (.stm) file to read and write data. When an IP client requests a download of messages using POP3 or IMAP4, the Exchange store that contains the user's mailbox will calculate the exact size of the messages in the Inbox to send to the client before transmitting the messages. In large environments, especially hosted Exchange environments, this can take a considerable amount of time to calculate. The Compatibility value represents a configuration opportunity for administrators to instruct the Exchange store to use an approximate message size instead of exact calculations for message sizes.

The Compatibility registry value is available for the Exchange POP3 service, as described in this article, and for the Exchange IMAP4 service, as described in The IMAP4 Compatibility flag has been set in the registry.

The Compatibility registry value is also useful during a move of a large number of mailboxes, or mailboxes with a large number of Multipurpose Internet Mail Extensions (MIME) messages. When moving data, the data is promoted from the streaming store into the MAPI store (.edb file). Clients that use POP3 and IMAP4 to access their mailboxes may experience performance issues after you move their mailboxes between databases or servers. After the user logs on, message sizes must be calculated and MAPI-to-MIME conversion occurs in memory and on the disk of the server. In extreme cases, very large temporary files may be created on the Exchange Server computer when this behavior occurs.

Not all POP3 and IMAP4 mail clients may work correctly with the Compatibility registry value set. Changing this setting causes some older mail clients (such as Fetchmail) to break. Setting this registry key also breaks RFC-compliance for the POP3 protocol. If you are experiencing problems downloading messages with POP3 and IMAP4 and you are using the Compatibility registry value, you can delete it using the procedure below as a troubleshooting step for the message download problems.

This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore the registry if a problem occurs. For information about how to restore the registry, view the "Restore the Registry" Help topic in Regedit.exe or Regedt32.exe.

To delete the Compatibility registry value

  1. Open a registry editor, such as Regedit.exe or Regedt32.exe.

  2. Navigate to: HKLM\System\CurrentControlSet\Services\POP3Svc\Parameters

  3. Delete the value called Compatibility.

  4. Exit the registry editor and restart the Microsoft Exchange Information Store service and the IIS Admin service for the change to take effect.

Before you edit the registry, and for information about how to edit the registry, see the Microsoft Knowledge Base article 256986, "Description of the Microsoft Windows Registry" (

For more information about the use of the Compatibility registry value, see the following Knowledge Base articles: