With modern mail clients you will have the ability to send the mail content as html. I a time where mails are read on all kind of devices with different screen sizes you might face the need for a more advanced web/html design that you might not be able to obtain in your mail client.
Making your responsive web design in a 3rd party application you ends up with your perfect design (e.g. for your newsletter) as a html file with a simple last task in mind - "Now I just have to mail it". At this point you might find out that this simple task might not be as simple as you expected.

MailMyHtml to your resuce, saving you for hours of googling to find a solution.
Simply MailMyHtml enables you to import your html file from Dropbox and put it directly in the mail body - ready to be send.

If you do not have an existing html file, you also have the option to write html directly in the app and preview it before sending it.