In Active Directory when you have an Exchange server a user or group, even a public folder can have multiple smtp email addresses associated with the entity (user/group/public folder etc.), but finding the non-default email address can be tedious. A great way to search for it is:
Go to Active Directory Users and Computers –> right click on the domain and click find –> in the find field, change the criteria to ‘custom search’ –> click the advanced tab where you can type in a LDAP query. If you are trying to find out who has email@example.com, type proxyAddresses=smtp:firstname.lastname@example.org.
Wildcards can be added to the email address portion, and you can also type a wildcard ie. *.yourdomain.com and then change the view to add the proxyaddress column to see all of the entities with their proxyAddresses (email addresses)