Abort sending Email

ext_localconf.php

<?php

/** @var \TYPO3\CMS\Extbase\SignalSlot\Dispatcher $signalSlotDispatcher */
$signalSlotDispatcher = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Extbase\\SignalSlot\Dispatcher');
$signalSlotDispatcher->connect(
	'In2code\\Powermail\\Domain\\Service\\Mail\\SendMailService',
	'sendTemplateEmailBeforeSend',
	'In2code\\Powermailextended\\Domain\\Service\\SendMailService',
	'manipulateMail',
	FALSE
);

SendMailService.php

<?php
namespace In2code\Powermailextended\Domain\Service;

use In2code\Powermail\Domain\Service\Mail\SendMailService as SendMailServicePowermail;

class SendMailService {

	public function manipulateMail($message, &$email, SendMailServicePowermail $sendMailService) {
		if ($email['template'] == 'Mail/OptinMail' &&  $email['subject'] == 'subject line') {
			// Never send such Optin mails ...
			$email['send'] = false;
			return;
		}
	}
}