RT_SiteConfig_RT_AUTHEN

   Set($ExternalAuthPriority,           [ 'My_LDAP' ] );
   Set($ExternalInfoPriority,           [ 'My_LDAP' ] );
   Set($ExternalServiceUsesSSLorTLS,    0);
   Set($AutoCreateNonExternalUsers,     0);

   Set($ExternalSettings,      {
                        'My_LDAP'               =>  {

                        'type'                  =>  'ldap',
                        'server'                =>  'IP of DC',
                        'user'                  =>  'our_domain\our_user', #We were using 'cn=user,cn=RT-OU,dc=domain'
                        'pass'                  =>  'our_user_passwd',
                        'base'                  =>  'dc=ourdomain,dc=com',

                        'filter'                =>  '(&(ObjectCategory=User)(ObjectClass=Person))',
                        'd_filter'              =>  '(userAccountControl:1.2.840.113556.1.4.803:=2)',

                        'tls'                   =>  0,
   #                    'ssl_version'           =>  3,

                        'net_ldap_args'         => [    version =>  3           ],
                       # 'group'                 =>  'cn=RT-Sec-Grp,ou=RT-OU,dc=our_domain,dc=com', <-- commented out this line, we though this may have been the issue
                       # 'group_attr'            =>  'member',

                        'attr_match_list'       => [   'Name', 'EmailAddress'   ],
                        'attr_map'              => {   'Name' => 'sAMAccountName',
                                                       'EmailAddress' => 'mail',
                                                       'Organization' => 'physicalDeliveryOfficeName',
                                                       'RealName' => 'cn',
                                                       'ExternalAuthId' => 'sAMAccountName',
                                                       'Gecos' => 'sAMAccountName',
                                                       'WorkPhone' => 'telephoneNumber',
                                                       'Address1' => 'streetAddress',
                                                       'City' => 'l',
                                                       'State' => 'st',
                                                       'Zip' => 'postalCode',
                                                       'Country' => 'co'
                                                   }
                                                   }
                               }
   );