Hi All, I've been researching this and I think I've just confused myself more than anything

I have a couple of clients who are thinking of doing some attractive html email newsletters (one already sends a plain text version and one sends a pdf). I am happy to use MailChimp or Campaign Monitor but ideally would like to be able to prepare my own template and mail it out (depending on numbers of course).
I have Dreamweaver and I have previously done a template for Campaign Monitor that required a little tweaking, but seemed to work okay, but what I'd like to know is if there is a email program out there that works easily with an html template from something like Dreamweaver? I currently use Thunderbird and the methods described don't work (although if I attach an html file it appears in the inbox nicely?!?). I also use a Mac, so that might narrow my options.
Does anyone know of any good tutorials/programs/articles that may help me learn how to set up/format my own html newsletter and just email it from an email client? I'm just trying to save my clients money (although Campaign Monitor is so cheap it's hardly an issue) and teach myself something useful and challenging.
Any suggestions? Thanks heaps in advance!
Marie