Overview of Managing Mail Contacts and Mail Users

Mail Contacts
          Mail contacts are mail-enabled AD DS contacts, that contain information about people or organizations that exist outside your Exchange Server organization. You can view mail contacts in the GAL and other address lists, and you can add them as members to distribution groups.
Each contact has an external email address, and all email messages that are sent to a contact are
automatically forwarded to that address.
          Create a mail contact with that person’s email address if multiple people within the organization contact a trusted external person, it allows Exchange Server users to select that person from the GAL for sending email.

Mail Users
          Mail users are have external email addresses; both contain information about people outside your Exchange Server organization, and both can be displayed in the GAL and other address lists. Mail users also have AD DS logon credentials and a security identifier (SID) that enable them to access network resources to which they are granted permission.
User external to your organization requires access to resources on your network, you should create
a mail user instead of a mail contact for that individual.