The Tip of the Night for June 23, 2016 discussed the familiar X.400 email format. This email format includes multiple fields which gives you additional information about the user:
John Smith <"c=US/o=Acme/ou=corp/s=Smith/g=john">
This is different than the standard SMTP email format: email@example.com . X.400 was an email standard developed by the ITU Telecommunication Standardization Sector. ITU originated as the International Telegraph Union, and has established telecommunication standards of many kinds. The SMTP email standard was set by the Internet Engineering Task Force (IETF).
ITU also developed the X.500 email format, which has a directory structure similar to Active Directory. In contrast to X.400, X.500 will have a format like this:
o=OldCOmpanyName 1/ou=First Administrative Group/cn=Recipients/cn=Firstname.LastName
X.400 and X.500 email format are commonly found in messages exchanged between users on the same Exchange server. The administrative group for a X.500 email should be identified by an alphanumeric string. If you know the group code for a server, and you've collected email messages from custodians which all shared that server, searching for the group code is an easy to determine if you have any X.500 emails which might elude standard searches for email addresses.
A post here, shows how an X.500 email address is displayed in Microsoft Exchange.