Solving “e-mail address already exists in this organization” problem when you cannot find the owner of an email address.
When creating an e-mail address for a user in active directory, you get the message “This e-mail address already exists in this organization. ID no: c10312e7 Microsoft Active Directory – Exchange Extension”
Finding email addresses with LDIFDE or CSVDE, only exports user and contact details. If the email address you are looking for is used by a public folder, the it will not show up in the export.
Heres an easier and faster way:
Using the Active Directory Users and Computers utility, right click on the domain, choose Find. In the Find: dropdown list, choose “Custom Search” and in the In: dropdown list, choose “Entire Directory”. Click the Advanced property page and enter the following LDAP query: “proxyaddresses=smtp:firstname.lastname@example.org”