Can I get a copy of the page you are working on? I'll test and see if I can reproduce it.
I see the edit with Dreamweaver button doesn't work for me, but the edit seems to work in a new application. Perhaps it is an issue with your page, or perhaps it is a MAC issue since I'm using a PC. If you send me your file I'll look into it.
I'd suggest moving to getting the content for the email from a page. Open the editor and copy the content. (escape key should get you out so you don't need to force quit). Then create a blank page with the content and when you open the UE server behavior select to get the contents from a file and browse to the file you created. Then you can easily edit the email body from that file directly moving forward until this issue is resolved.