Resources/Private/Templates/Mail.html
<html xmlns="http://www.w3.org/1999/xhtml" lang="en"
data-namespace-typo3-fluid="true"
xmlns:f="http://typo3.org/ns/TYPO3/Fluid/ViewHelpers">
<body>
<f:layout name="Default"/>
<f:section name="Main">
<f:format.raw>{message}</f:format.raw>
</f:section>
</body>
</html>