Currently, forum posts that are being sent via HTML mail contain the HTML code, but no styling information. This is an issue itself and is not easy to solve.

A problem users are being annoyed by most however is the displaying of attachments. Here the file icon is (due to the lack of the corresponding icon css class) being displayed as huge icon. As quick fix I suggest to wrap the icon in a container to at least keep this pain point away from the users until there is a general solution to the missing stylesheets in HTML mails.