In the explanation of Table 5 in the paper (page 21), there's a list of attack vectors with the title "HTML attributes (bypasses for remote content blocking)".

In the explanation of Table 5 in the paper (page 21), there's a list of attack vectors with the title "HTML attributes (bypasses for remote content blocking)". So apparently some of these attributes do bypass the inline content blocking on some mail clients, unless I read that wrong. It's important to note that most e-mail clients will use a webview to render e-mail. Blocking all kinds of remote content requires the developers to be really careful, and also probably violates the HTML spec. I tried to replicate the Thunderbird attack vector (H₂), but at least in the current version (52.7.0) it seems that the URL is not requested. Maybe that has been fixed in the meantime but was present in an older version? Disabling HTML rendering for now is probably the safest choice. There's not much that can go wrong when rendering a plaintext e-mail. (Edit: Added a sentence regarding webviews)