A mail-enabled object has an e-mail address associated with it but no mailbox, and thus it can receive messages but not send them in Exchange.
Use Active Directory Users and Computers to enable e-mail capabilities for a user, group, or contact.
A mailbox-enabled object is an object that has an Exchange mailbox associated with it, and thus it can both send and receive e-mail messages.
Only Windows user accounts can be mailbox-enabled.
Use Active Directory Users and Computers to enable a mailbox for a Windows user account.