Outlook will by default block several attachments including .mdb files, there is no way from within outlook to unblock these attachments, however what you can do is insert a new string value of Level1Remove with data value consisting of the file extensions that you wish to unblock such as .mdb;.pst;.etc into My Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security
see http://www.hanselman.com/blog/UnblockingBlockedAttachmentsInOutlook.aspx
