ONLY invited developers can have access to the app, because of the way the Mail API of GAE works. If you are not an invited developer, you can access the source code (see below) to deploy it on GAE under your own credentials.
Documentation: https://docs.google.com/file/d/0B-eJZhiLa8wSU1NsMGRSRy1vdFE/edit
Source code: https://docs.google.com/file/d/0B-eJZhiLa8wSTUlJSTdGWlc4RE0/edit
Brief description:
1. Get a Google Doc as a template, which has variables starting with '$'
--> These variables should NOT be the last word of the document
2. Merge it with a Google spreadsheet which has a first row the names of the columns
--> This Spreadsheet should have a column called 'subject' and another called 'to', which are going to be used for the recipients and the subjects of the emails correspondently.
3. ... And, send personalized emails using the template filled with the values of the corresponding rows of the spreadsheet.
Extore is a team of professionals who are passionate about creating extensions for web browsers. This devotion also gives us opportunity to appreciate work of other people. We get inspired by useful, open source extensions made by developers all over the world. Our strong belief is that one should share helpful add-ons with others. That’s why we’d like to present you our ever-growing list of favourite extensions that have inspired us. Moreover, as you probably have already learned, sometimes struggling for the best brings an end to the good. Talking about add-ons it often turns out that an upgrade is worse then previous version. Also it can become really tricky to get back to the beloved version of your favourite extension. That’s why we’re going to make not only the latest, but all versions of our favourite (and we hope yours too) add-ons available for download. No more compromises, just stick to the version you really like!