I built a similar tool back in 2010 - for gmail, yahoo mail and hotmail.
It was a browser plugin for Chrome, Firefox and Internet Explorer.
I learned a lot about Gmail's DOM (it's highly obfuscated) and how terrible the plugin-writer's experience is for Internet Explorer.
After six months of promotion on peoplesmart.com as an add-on to allow our users to get more information about the people in their email conversations, we had about twenty users total.
The project was scrapped, and I moved on to my next thing.
It was a browser plugin for Chrome, Firefox and Internet Explorer.
I learned a lot about Gmail's DOM (it's highly obfuscated) and how terrible the plugin-writer's experience is for Internet Explorer.
After six months of promotion on peoplesmart.com as an add-on to allow our users to get more information about the people in their email conversations, we had about twenty users total.
The project was scrapped, and I moved on to my next thing.
Perhaps it was just timing. Best of luck!