Perfect!

The Plugin is perfect! Everything you need, is available.

Some sugestions:

Basic-Setting:
Make it easier for users to fill in different kinds of Emails:
Instead of filling in Tags and codes and semicolons etc. offer a Input field like it is already for Bcc.
There are often error-messages, when filling in above mentioned stuff in the big field.

Styles:
Its very tricky to formate and style all the fields and functions with all that code-stuff. A lot of time will be spended in try-and error untill one has finally his layout done.
Please insert color pickers and some preview of what happens on changes to get this more visually.

Continue with your project – Its great!