I think #6781 is more about the tracking and reporting part than providing an actual hosted newsletter application. Also one problem would be that it uses Nodejs whereas Piwik is based on PHP. As Piwik is running on hundred of thousands of servers Nodejs will be often not available and most users won't be able to install it. Users will still need to install it separately. What kind of email marketing features are you thinking of?
BTW: A plugin could be developed to connect to Mailtrain. There are many integrations already available see http://piwik.org/integrate/ and it would be useful to connect to Mailtrain or to send data from Mailtrain to Piwik