[kh99]

You could do something like this: create a plugin using hook pmdata_presave and code like:

PHP Code:

foreach ($this->info['recipients'] as $u)
{
    $user = fetch_userinfo($u['userid']); // Note: need to do this because $this->info['recipients'] does not contain profile fields
    if (some check of fields in $user)
    {
        $this->errors[] = "Can't send PM to user " . $user['username'];
    }
}