You can use the languageList configuration option to change the "Regarding image" and "in gallery" text in the email body but it is not possible to completely change the email body text.
languageList is used primarily to translate gallery text into a different language but you can change the text to whatever you like.
Please see the International Gallery Text support section for details.

"Regarding image" and "in gallery" are the last two text strings in the languageList so, if you wanted to change "Regarding image" to "abc" and "in gallery" to "123" (for example), then you would use the following languageList:

languageList="Show Thumbnails|Hide Thumbnails|Expand Gallery|Close Gallery|Open Image in New Window|Images|Next Image|Previous Image|Play Audio|Pause Audio|Show Information|Hide Information|Start AutoPlay|Stop AutoPlay|AutoPlay ON|AutoPlay OFF|Go Back|Buy this Image|Share on Facebook|Share on Twitter|Share on Google+|Share on Pinterest|Share on Tumblr|of|Send Email|Download|Password|Incorrect Password.|abc|123"

You would need to add languageList to your gallery's configuration options manually (please see here for details) as languageList is not featured in the JuiceboxBuilder-Pro interface.

Edit:
You could override Juicebox's own handling of the Email Button and add a custom click handler to the button to perform your own functionality (e.g. set up an email with your own body content).
Try something like this:

<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        backgroundColor: "#222222",
        containerId: "juicebox-container",
        galleryHeight: "400",
        galleryWidth: "600",
        showEmailButton: "TRUE"
    });
    jb.onInitComplete = function() {
        $('.jb-bb-btn-email').off('click');
        $('.jb-bb-btn-email').on('click', function() {
            var index = jb.getImageIndex();
            var address = 'email@address.com';
            var subject = 'Gallery';
            var body = 'Image No. ' + index;
            window.location.href = 'mailto:' + address + '?subject=' + subject + '&body=' + body;
        });
    };
</script>
<div id="juicebox-container"></div>

Just change the body text in the line:

var body = 'Image No. ' + index;

... to whatever you like, e.g:

var body = 'This is the email body text.';

Please note that although you are free to use Juicebox as you wish (within the Terms of Use), modifications such as this (overriding core Juicebox functionality) are not officially supported.