Unfortunately, it is not possible to change the Email Button's body text. The text is buried deep within the 'juicebox.js' file which is obfuscated and cannot be modified.
We plan to make it possible for users to translate "Regarding image" and "in gallery" (via the languageList configuration option) in the next version of Juicebox-Pro but, as far as I am aware, there are no plans to allow the entire text to be substituted.

The best I can suggest at the moment is that you either:
(1) Suggest your idea (to be able to replace the Email Button's body text) for a future version in the Feature Requests forum thread. This keeps all the ideas together and ensures that they are not overlooked by the developers.
... or:
(2) Override the Email Button and create your own 'mailto' link (with your own custom body text) as suggested in this forum post. (Please note that modifications such as this which override Juicebox's own built-in functionality are not officially supported and while you are free to try them, you might find that you encounter unforeseen knock-on problems which might need to be tackled along the way.)