Assuming you mean Windows Server 2003’s built-in NTBackup utility. NTBackup doesn’t do brick level (that is, individual mailboxes) backup. Brick level backups use MAPI to connect to the Exchange Store, are terribly inefficient and <b>less than fashionable</b>.
Exchange Server 2007 has better functionality, including Recovery Storage Groups for disaster recovery scenarios. Many third-party Exchange backup utilities can recover individual mailboxes from a Store backup.