Mail-Enabling Versus Mailbox-Enabling
Exchange has two primary mechanisms for hooking users into the mail system. The first mechanism is called mailbox-enabling and associates a mailbox in the Exchange organization to a user. This is the normal setting for users in most organizations that allows them to log into the Exchange system with Outlook, or Outlook Web Access (OWA), or other mail clients that can communicate through standard mail interfaces such as POP3 or IMAP.
The second mechanism is called mail-enabling; this connects one or more email addresses to the user that are external to the Exchange organization. This is used for someone who needs to log into the forest but does not want a mailbox in the Exchange organization. For example, Dr. Amy Gramzow of the Acme Consulting Company could be on a long-term assignment at MyCorp, Inc. and have any email sent to her DrAmy@MyCorp.com email address forwarded on to her AGramzow@AcmeConsulting.com mailbox so that she doesn't have to manage multiple mailboxes.
Mail-enabling is also used to make Exchange aware of contacts and groups. These objects can't be authenticated, so creating a mailbox for them doesn't make sense; they must be mail-enabled instead of mailbox-enabled. Contacts will almost always have an external email address associated with them; a group, on the other hand, will be used as a distribution list to forward messages onto multiple recipients, which could be internal to the organization or outside of it.