Event ID : 9549

Issue : Not able to send email to a perticular mailbox OR not able to open mailbox using OWA.

Event Source: MSExchangeIS

Cause : This issue may occur if another account exists that has the same e-mail address. The same SMTP proxy address has been assigned to more than one object in Microsoft Windows 2000 Active Directory. Event 9514 indicates which objects have the same SMTP address.

In Exchange 2000, you may experience one of the following symptoms:

1. When you try to send a message, you receive a non-delivery report (NDR) that includes an "ambiguous recipient" error message or a Simple Mail Transfer Protocol (SMTP) delivery status notification code 5.1.4.
2. You receive the following message when you add a proxy address to a user account:
   This e-mail address already exists in this organization. ID no. c10312e7 Microsoft Active Directory - Exchange Extension.

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.

Action Required :

1. To resolve this issue, locate the user account that has an ambiguous address or an address that already exists (to find the address, view the E-mail Addresses tab in the user account properties), and then change one or more of the accounts so that they do not have the same e-mail address.To locate the user account that has an ambiguous address or an address that already exists, use either of the methods that are described in this section.
2. To resolve this problem, find the objects that have duplicate SMTP addresses using event 9514, and then modify the SMTP address.

Ref Link:
http://support.microsoft.com/kb/286792/en-us

http://support.microsoft.com/kb/269498/en-us