When you search Active Directory for an email address, you will only get a response if the address you are searching for is the Primary SMTP address.
To search for other addresses, whether they are attached to a user, contact or public folder, follow the steps below;
1. Login to the Exchange Server and open Active Directory Users and Computers
2. Right click on the domain and choose Find
3. In the find field, change the criteria to "Custom Search"
4. Click the Advanced tab so you can enter an LDAP query
5. Type proxyAddresses=smtp:firstname.lastname@example.org
6. Click find to search