Riporto direttamente da http://www.danlance.co.uk/index.cfm/2007/11/14/How-to-find-the-user–entity-an-email-address-is-associated-with-in-Exchange-2003

* Open the Acive Directory Users and Computers Management Console
* Right Click on the domain > Select Find
* Select Custom Search in the Find field
* Select the Advanced tab
* To find the entity with the email address spamtrap@danlance.co.uk for example, enter the following in the LDAP query field:
(proxyAddresses=smtp:spamtrap@danlance.co.uk)
* This will list the entity containing the email address entered.

* Wild cards can be entered within the email address (*) to return multiple matches.
* If you want to view all the email addresses within the results list, select View > Choose Columns… and select Proxy Address – doing this in assoication with the query (proxyAddresses=smtp:*@danlance.co.uk) will give you a list of all entities within the organisation that have an email address, together with all the email addresses they have.