If you are considering utilizing a Java backend or are open to deploying on Google App Engine, you may want to explore the possibilities with Google Web Toolkit. They offer a wealth of tutorial resources and sample applications, including a Mail application that could potentially be customized to meet your specific requirements.
If GWT is not your preferred option, there are numerous JavaScript UI Libraries available for use alongside any backend system; such as YUI, Ext, jQuery UI, MochaUI, and others. Alternatively, you can create your own user interface utilizing foundational cross-browser libraries (YUI, Ext JS, jQuery, MooTools, etc.) or simply traditional JavaScript/HTML/CSS/DOM.
The choice ultimately depends on your specific needs, desired level of code control, aesthetics, behavior, and personal preferences.
It is advisable to enhance your understanding of JavaScript fundamentals, Object Oriented JavaScript, HTML/CSS, and server-side language, in order to develop applications with structured, clean, and sustainable code. There are plenty of instructional resources available on this topic.
In reality, locating a tutorial that aligns with your chosen development stack and expanding from there would be most beneficial.
Striving for a balance between the two approaches would likely yield optimal results.