framework/mail/BaseMailer.php
public function sendMultiple(array $messages)
    {
        $successCount = 0;
        foreach ($messages as $message) {
            if ($this->send($message)) {
                $successCount++;
            }
        }

        return $successCount;
    }
public function sendMultiple(array $messages)
    {
        $successCount = 0;
        foreach ($messages as $message) {
            if ($this->send($message)) {
                $successCount++;
            }
        }

        return $successCount;
    }
Sends multiple messages at once.

The default implementation simply calls [[send()]] multiple times.
Child classes may override this method to implement more efficient way of
sending multiple messages.

@param array $messages list of email messages, which should be sent.
@return int number of messages that are successfully sent.