Magento – Magento doesn’t send autogenerated password

Take a look at Magento Migrate customers with no password.

$customer = Mage::getModel('customer/customer');

$password = '123456';
$email = 'testuser@test.com';

$customer->setWebsiteId(Mage::app()->getWebsite()->getId());
$customer->loadByEmail($email);

if($customer->getId()) { // if customer does not already exists, by email

    $newPassword = $customer->generatePassword(); // generate a new password
    $customer->changePassword($newPassword); // set it
    $customer->save();    
}

Assuming that you want to use the default magento password template then take a look at app/code/core/Mage/Adminhtml/controllers/CustomerController.php

// Send welcome email
if ($customer->getWebsiteId() && (isset($data['account']['sendemail']) || $sendPassToEmail)) {
    $storeId = $customer->getSendemailStoreId();
    if ($isNewCustomer) {
        $customer->sendNewAccountEmail('registered', '', $storeId);
    } elseif ((!$customer->getConfirmation())) {
    // Confirm not confirmed customer
            $customer->sendNewAccountEmail('confirmed', '', $storeId);
    }
}

From Outside Magento.

If you want to send All Customers Reset Password Email, then below is the code.

//place this before any script you want to calculate time
$time_start = microtime(true);

// MAGENTO START
include('app/bootstrap.php');

use Magento\Framework\App\Bootstrap;
use Magento\Customer\Api\AccountManagementInterface;
use Magento\Customer\Model\AccountManagement;

$bootstrap = Bootstrap::create(BP, $_SERVER);
$objectManager = $bootstrap->getObjectManager();

$state = $objectManager->get('Magento\Framework\App\State');
$state->setAreaCode('frontend');

$_customers = $objectManager->create('Magento\Customer\Api\AccountManagementInterface');
$customerCollection = $objectManager->create('Magento\Customer\Model\ResourceModel\Customer\Collection');
$customerCollection->load();
$i = 0;
foreach ($customerCollection as $customers) {

    $email = $customers->getData('email');

    try {
            $_customers->initiatePasswordReset($email, AccountManagement::EMAIL_RESET);
        } catch (NoSuchEntityException $e) {
            // Do nothing, we don't want anyone to use this action to determine which email accounts are registered.
        } catch (\Exception $exception) {
            echo __('We\'re unable to send the password reset email.');
        }
        echo "<pre>";
        echo $i . " Email :-" . $email . " Mail Sent";
    $i++;
}

echo "<pre>";
echo "Total Customers: " . $i;

$time_end = microtime(true);

//dividing with 60 will give the execution time in minutes other wise seconds
$execution_time = ($time_end - $time_start) / 60;

//execution time of the script
echo "<pre>";
echo 'Total Execution Time:</b> ' . $execution_time . ' Mins';

Hope it helps Magento Community :)