MockSMTP.app - e-mail testing for Mac OS X
mocksmtpapp.com
mocksmtpapp.com
For instance, MailChimp takes CSS styles and modifies your HTML by placing your CSS into inline style tags. This is great, because you can have cleaner markup but still support clients that don't apply CSS well.
(I'm not affiliated with them in any way, just found the app to be very useful while working on a recent project)
It's like make HTML/CSS for a webpage. First level, you create you HTML/CSS code and test it with Firefox usually. Then when you are happy with, you check safari/chrome and debug IE :).
1/ it works offline (think git vs svn) - so you can work in a train for instance
2/ if you test your app with real data (ie: many different emails), you're sure no email will go out by mistake
3/ you can use as many recipients as you want (with any kind of address) and see the emails right away
I could be wrong though - my understanding!