E-mail clients have a very limited support of CSS. In most cases, they don't support linked stylesheets. Inline stylesheets are often not supported as well. Inline styles are better understood.
This utility, based on Emogrifier, will “automagically transmogrify your HTML by parsing your CSS and inserting your CSS definitions into tags within your HTML based on your CSS selectors”.
It is configured by TypoScript, and is to be applied on pages used to prepare Direct Mails.