FluidMailMessage.php

<?php
namespace Smichaelsen\Gist\Mail;

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Extbase\Mvc\Controller\ControllerContext;

/**
 * This class represents an email which can be rendered with fluid.
 * You can use all the possibilities of TYPO3's MailMessage but instead of
 * using ->setBody() and ->send() you rather use ->render($template) to
 * render and send a fluid template.
 * Before rendering you can ->setControllerContext() and ->assign() variables.
 */
class FluidMailMessage extends \TYPO3\CMS\Core\Mail\MailMessage {

	/**
	 * @var \TYPO3\CMS\Extbase\Mvc\Controller\ControllerContext
	 */
	protected $controllerContext;

	/**
	 * @var \TYPO3\CMS\Fluid\View\StandaloneView
	 * @inject
	 */
	protected $view;

	/**
	 * @param ControllerContext $controllerContext
	 */
	public function setControllerContext(ControllerContext $controllerContext) {
		$this->controllerContext = $controllerContext;
		$this->view->setControllerContext($controllerContext);
	}

	/**
	 * Assigns a key value pair to the fluid template
	 *
	 * @param string $key
	 * @param mixed $value
	 */
	public function assign($key, $value) {
		$this->view->assign($key, $value);
	}

	/**
	 * See ->setTemplate() for possibilities to express the template path
	 *
	 * @param string $template
	 */
	public function render($template) {
		$this->setTemplate($template);
		$content = $this->view->render();
		$this->setBody($content);
		$this->send();
	}

	/**
	 * You can either provide a absolute template path or a template path
	 * beginning with 'EXT:'.
	 * If you have provided a ControllerContext (->setControllerContext()) you
	 * can also just provide the file name when your template lies in
	 * EXT:yourext/Resources/Private/Templates/Mail/
	 * And finally you can also provide the full template source as a string.
	 *
	 * @param string $template
	 */
	protected function setTemplate($template) {
		$possibleFullTemplatePaths = array(
			GeneralUtility::getFileAbsFileName($template),
			$template,
		);
		if ($this->controllerContext instanceof ControllerContext) {
			$possibleFullTemplatePaths[] = ExtensionManagementUtility::extPath($this->controllerContext->getRequest()->getControllerExtensionKey()) . 'Resources/Private/Templates/Mail/' . $template . '.html';
			$possibleFullTemplatePaths[] = ExtensionManagementUtility::extPath($this->controllerContext->getRequest()->getControllerExtensionKey()) . 'Resources/Private/Templates/Mail/' . $template;
		}
		$templateIsFile = FALSE;
		foreach ($possibleFullTemplatePaths as $possibleFullTemplatePath) {
			if (file_exists($possibleFullTemplatePath)) {
				$this->view->setTemplatePathAndFilename($possibleFullTemplatePath);
				$templateIsFile = TRUE;
				break;
			}
		}
		if (!$templateIsFile) {
			$this->view->setTemplateSource($template);
		}
	}

}

I wouldn't call ->send() within render(), I feel like it's not in it's responsibility... Other than that this is great!

This class is now available as composer package: smichaelsen/typo3-fluidmailmessage