You do not receive a warning message when you specify a duplicate e-mail address in the Active Directory Users and Computers MMC snap-in on a Windows Server 2003-based computer that has Exchange System Management Tools installed

Consider the following scenario. You install Microsoft Exchange System Management Tools for Microsoft Exchange Server 2003 on a Microsoft Windows Server 2003-based computer. In the Active Directory Users and Computers Microsoft Management Console (MMC) snap-in, you specify an e-mail address for an object. However, the e-mail address that you specify is already used by another object. In this scenario, you do not receive a warning message when you specify this duplicate e-mail address.

Additionally, this problem occurs when you use the Find feature in the Active Directory Users and Computers MMC snap-in to locate the object for which you specify the e-mail address.

This problem occurs because the Active Directory Users and Computers MMC snap-in does not generate a notification when you specify the e-mail address for an object in Active Directory that you select in the Find dialog box. Therefore, Exchange Extension (Maildsmx.dll) does not check for uniqueness even though the new e-mail address is a duplicate.

To work around this problem, use the Active Directory Find feature to make sure that the e-mail address that you specify is not used by any other object. Do this when you want to change the e-mail address for an object.

The expected warning message is raised by Maildsmx.dll to the Active Directory Users and Computers MMC snap-in. Maildsmx.dll generates the warning message only when the management console notifies Maildsmx.dll that an e-mail address has changed. When Maildsmx.dll is notified, it checks the new e-mail address for uniqueness in Active Directory. If Maildsmx.dll detects a duplicate e-mail address, Maildsmx.dll raises the warning message.

When you create a duplicate e-mail address for an object, the following warning message is supposed to appear:This warning message does not prevent you from creating the duplicate e-mail address. However, you can take appropriate actions when you receive this warning message.

Additionally, the following problems may occur if you continue to use the duplicate e-mail address for the object in Active Directory:

• E-mail messages that you send cannot be delivered to the objects that share this e-mail address. In this scenario, the you receives the following non-delivery report (NDR):
  A configuration error in the recipient's e-mail system prevented delivery of this message. Two recipients are configured with the same e-mail address. Contact your administrator.
• Every time that you send an e-mail message to one of the objects that use this e-mail address, an event that resembles the following is logged in the Application log:

  Event ID: 9514
  Event Type: Warning
  Event Source: MSExchangeIS
  Description:
  Two objects in the Directory have the same proxy
  /DC=COM/DC=[domain]/CN=[Container1]/CN=[Object1] and
  /DC=COM/DC=[domain]/CN=[Containe2]/CN=[Object2]

• If you are a mailbox-enabled user who has a duplicate e-mail address, you receive the following error message when you try to log on to the mailbox in Microsoft Outlook Web Access (OWA):
  
  HTTP 500 - Internal server error
• An event that resembles the following is logged in the Application log:

  
  Event Type: Error
  Event Source: MSExchangeIS
  Event Category: General
  Event ID: 9549
  Description:
  An ambiguous SMTP proxy user1@domain.com was found on 0x2 mailboxes in the DS. The store cannot map this SMTP proxy to a unique Mailbox GUID.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.