Elmar is asking about Dynamics 365 for Marketing emails, and very specific functionality that lets you send great looking marketing emails , and not the standard Email Templates.
I do not have the answer sadly, but I don't believe this is an out of the box feature available with Dynamics 365 for Marketing
And I am wondering about somewhat the same thing, having created a thread asking about transactional email. If sending emails through Dynamics 365 for Marketing from external sources would be supported, you could create the 'quick send' functionality yourself :)