[Logician]

in private.php, find:

-- cut ---
if (!$fromuserinfo['receivepm'] or !$fromusergroup['canusepm']) {
--- cut--

BEFORE that ADD:
--cut--
if ($bbuserinfo[posts]<XX) { show_nopermission();exit;}
-- cut--
(Dont forget to replace XX with minimum message number)

And add the same line, before this line too:
-- cut ---
if ($message=="" or $touser=="" or $title=="") {
-- cut--


Not tested, should work though.. If not, holler and I'll fix it..

Enjoy! \\=^))

Logician