Here are a few ways how to find who is using a particular email address in your organization.

Microsoft Outlook

• Open MS Outlook > Create a new message, start typing email address and press CTRL + K
  Email should resolve to the recipient name. Only works with recipients present in Global Address List.

Active Directory Search

• Open Active Directory Users and Computers
• Select your domain name > click on Find Objects… > select Custom Search > Advanced
• In Enter LDAP query field type:
  (proxyAddresses=smtp:email@domain.com)
  Press Find Now
  You can also use wildcards, i.e. (proxyAddresses=smtp:email*)

Exchange Management Shell (Exchange 2007 and 2010)

• Run EMS cmdlet:
  get-recipient | where {$_.emailaddresses -match “email@domain.com”} | select name,emailaddresses | fl