Take a look to li3_mailer plugin.
From the docs, an answer to your question :
Rendering email messsages is similar to rendering responses with a few exceptions. The most important is that instead of having a request which can be negotiated to infer the most suitable (single) type for response the email message may have multiple types (and does not have a 'corresponding' request). To support this the plugin implements a mail Media class simlar to http's Media, which can be used to register new types or configure the built-ins.