http://support.microsoft.com/default.aspx?scid=kb;en-us;822447

SUMMARY

This article describes how to modify recipient policies in Exchange 2003 to determine how a user's e-mail address is generated. When you use the procedure that is described in this article to modify a recipient policy, you can define specific settings that allow you to meet the naming conventions that are required for your organization.
 

MORE INFORMATION

By default, Exchange 2003 has one recipient policy that is named "Default Policy.” Default Policy generates Simple Mail Transfer Protocol (SMTP) e-mail addresses in the following format, where user_name is the Exchange alias property of the user, and domain_name is the Microsoft Active Directory directory service domain name:
user_name@domain_name.com
Note Do not modify Default Policy if you want to use the replacement strings that the following procedure describes to generate your SMTP address. If you want to use replacement strings, you have to create a new recipient policy. If you try to modify Default Policy, you may experience problems if Default Policy is based on attributes such as given name or surname. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
271339 (http://support.microsoft.com/kb/271339/EN-US/) XADM: Cannot Mount Database and Event ID 9546 Occurs