When running Microsoft Exchange, all the end-user's email addresses are stored in Active Directory. Some users can have multiple addresses in certain cases. Even groups can have multiple addresses as well. Sometimes you need to find which account gets mail for a certain address. Finding a non-default email address can be pretty time consuming and tedious if you were to right click each one and do properties, etcetera. A faster way is by running an LDAP Query. This can be done in Active Directory Users and Computers.
If you wanted to find out who has "firstname.lastname@example.org" in your domain you would enter this query in the "query" area: