Good suggestions and arguments for using a online tool / service here. But if you want to stick within your parameters of a Mac-based solution. Try:

Mail Designer Pro 2 by Equinux:
http://www.equinux.com/us/products/maildesigner2/mail-designer-pro-2/

Free trial and available on the App Store.

Completely wysiwyg, it does previews and customizations for everything from desktop to Apple Watches. And it is particular good if you use MailChimp or Campaign Monitor as you can add their template variables directly; and, there is one-click pushing of your finished email to their respective servers for distribution. You can also output html for other providers; and even PDFs for using elsewhere.

For emails you intend to send directly from Apple Mail itself (if you have a small list) you can use the little sister (the non "Pro" version) of the same program.

I have some minor issues with the program but find that for clients who really can't work with online tools it really steps up to the plate. There is even a "share / lend" via email feature for sending templates that auto install with a click. So I can design a custom template for a client that they can then customize at will.

If you have any follow up questions please DM me and I will be happy to share my VIP call link.