Interesting, but the problem with writing HTML e-mail is rarely real-browser testing, but rather, testing the convoluted filtering/rendering of the various webmail clients (Hotmail, Yahoo, GMail).
This is not the goal of mocksmtp. It's for first level testing. Checking html and correcting rendering with webkit. Then of course you need to test other mail/weblmail reader or with litmus as mention before.
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 :).
It would be great if this app provided some other functionality that helps with problems like that.
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.
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 :).