Today I was setting up a new email address for a user and I got the following error:

E-mail already exists in this organization

Apparently the address alreaty existed… how do I find it?

• Start Active Directory Users and Computers
• Goto Find…
• Select Custom Search
• Select the advanded tab
• put “proxyaddresses=smtp:mail.address@domain.com” into the editbox (drop the “s)
• Make sure the “Entire directory” scope is selected

Source

http://groups.google.com/group/microsoft.public.exchange.admin/browse_thread/thread/21888e98bb4e5ea9