Résolu,
pour roundcube 0.1.1 :
$rcmail_config['ldap_public']['LENOM'] = array(
'name' => 'LENOM',
'hosts' => array('yy.yy.y.yy'),
'port' => 3268,
'base_dn' => 'DC=xxxxx,DC=xx',
'bind_dn' => 'CN=nomdeuser,CN=Users,DC=xxx,DC=xxx,DC=xx',
'bind_pass' => 'password',
'search_fields' => array('mail','userPrincipalName','cn','sn','givenName'),
'name_field' => 'cn',
'firstname_field' => 'givenName',
'surname_field' => 'sn',
// 'email_field' => 'userPrincipalName',
'email_field' => 'mail',
'scope' => 'sub',
// 'filter' => '(&(mail=*)(!(msExchHideFromAdressLists=TRUE)))',
'filter' => '(&(mail=*@mondomaine))',
'fuzzy_search' => true
);