If you have a large Exchange environment with a large amount of users, contacts, distribution lists, and public folders you can end up “losing” email addresses. It can take a lot of time to perform the different types of searches required to locate an email address. Or you can use powershell and locate it quickly.
PS> Get-Recipient -ResultSize Unlimited | Where {$_.EmailAddresses -Match "bob@allanglesit.com"} | Select Name,EmailAddresses,RecipientType