Problem:
Did you ever searched an special email-address in your active directory? If yes, you will know, that there isn’t a special field for the email-addresses.
Solution:
Go to active directory users and computers (ADUC).
Right click on the domain and choose Find.
Select Custom Search in the Find-Field and Entire Directory in the In-Field.
Select the Advanced-Register and type this LDAP query:
proxyaddresses=SMTP:user@domain.com