Login to your inbox and open up the email you'd like to listen to. Use Read Selection to only read from the email's contents instead of everything else on the webpage.
Highlight the contents of the email and click on the Read Selection icon. Or, highlight the text, right-click and select 'Read selection'.
Also be sure to check your Read Selection settings. When using Read Selection, you can either have it only read the select text or to continue reading from the selected text to the end of the page.