Visit your [My Agency] tab >> click [Reports] sub-tab.
Since you want info on all the Cases (clients) you've entered select "Cases Report" type. See screenshot here: http://d.pr/i/B3YfM/4jqvHatS
On the 1st reporting screen: Select "CSV" format in the top-right corner -- You can filter further if wish but not necessary -- then click [Next] button. See screenshot here: http://d.pr/i/Df84ia/1XXzMyeJ
On the 2nd reporting screen: You can check the fields you need and un-check the fields you don't need, but be sure to include "Case: Email Address" and then click [Request Report] button. See screenshot here: http://d.pr/i/kRIILi/51kKMQE8
In a few minutes your CSV report will be ready to download (you'll receive an email notification) and then you can open the file in Excel and you'll find an "Email Address" column -- Feel free to delete any columns you don't need.