You start by picking a template; the application comes with a fair number of attractive presets and equinux has many more available for purchase. Your email is composed of Layout Blocks -- cleverly disguising tables -- in horizontal sections that you can add, remove and move up or down. The layout process is very fluid and generally quite satisfying if you don't need pixel perfect customization. While some parts of a Layout Block can be scaled, others don't seem to be scalable; the process often leaves you accepting a compromise if you really want to tweak every detail.
Presets are important throughout the application, while you will probably bring in your own photos and you have a lot of control over how text looks, Mail Designer includes some graphics, textures, text objects and image containers with varying degrees of quality for you to use out of the box. Unfortunately, many of these presets are also fairly limited in customizability; for instance, some objects can be scaled but only proportionally and images will need to be cropped to an image placeholder rather then adjusting the placeholder to the image.
Where Mail Designer shines though is letting you customize a template that you'll use repeatedly while just changing the content for each new email. It's perfect if you're writing a newsletter or find yourself sending out elaborate form-based emails on a regular basis but still need to be able to move sections around and insert or remove parts on a case-by-case basis.
The only thing that really surprised me was some strange non-standard window behavior with the application; for instance, hitting the Zoom button on the template window shrinks the entire window into a roughly 250 by 150px thumbnail that hangs out near the top left corner of my monitor instead of growing to fill the window. These strange behaviors are mostly harmless though and will probably never get in your way.
After a bit of testing Mail Designer truly does seem to achieve it's goal to make "mail design for everyone" a reality. However, people who want to send out attractive invitations and family newsletters may be turned off a little by the price. The app costs €59.95 (approximately US$87) which seems a little steep for casual users -- especially if you plan to buy any stationery packs -- but it seems more appropriately priced for small business users.
There is a trial available; it lets you get a good feel for the application but does not let you save or email without purchasing the full version. For power users it should give you a very good idea of whether the application offers enough customization for you before you hand over your credit card. If you're interested in finding out whether or not Mail Designer will produce output that works with your targeted list of mail clients, the company is hosting an online "Experts Day" next Tuesday morning where you can chat with the product support team.