http://vww6ybh7k4rgpbljii45mn4vxkr2qjzrrqxz7bamjm5iwwu3ofiqv6id.onion/en/lists/list-user/sending-mail
There are many possible vulnerabilities including session hijacking, session fixation, and tracking your activity via unique image URLs. Additionally, many of the most severe security flaws in Microsoft’s Outlook relate to HTML email, and there will no doubt be more.